Do not unfold the monadic lets for the generated F* code

15 files changed, 1667 insertions, 3632 deletions

diff --git a/tests/fstar/betree/BetreeMain.Funs.fst b/tests/fstar/betree/BetreeMain.Funs.fst
index 854862b2..8c0c1cc1 100644
--- a/tests/fstar/betree/BetreeMain.Funs.fst
+++ b/tests/fstar/betree/BetreeMain.Funs.fst
@@ -20,10 +20,8 @@ let betree_store_internal_node_fwd
   (id : u64) (content : betree_list_t (u64 & betree_message_t)) (st : state) :
   result (state & unit)
   =
-  begin match betree_utils_store_internal_node_fwd id content st with
-  | Fail e -> Fail e
-  | Return (st0, _) -> Return (st0, ())
-  end
+  let* (st0, _) = betree_utils_store_internal_node_fwd id content st in
+  Return (st0, ())
 
 (** [betree_main::betree::load_leaf_node] *)
 let betree_load_leaf_node_fwd
@@ -35,17 +33,12 @@ let betree_store_leaf_node_fwd
   (id : u64) (content : betree_list_t (u64 & u64)) (st : state) :
   result (state & unit)
   =
-  begin match betree_utils_store_leaf_node_fwd id content st with
-  | Fail e -> Fail e
-  | Return (st0, _) -> Return (st0, ())
-  end
+  let* (st0, _) = betree_utils_store_leaf_node_fwd id content st in
+  Return (st0, ())
 
 (** [betree_main::betree::fresh_node_id] *)
 let betree_fresh_node_id_fwd (counter : u64) : result u64 =
-  begin match u64_add counter 1 with
-  | Fail e -> Fail e
-  | Return _ -> Return counter
-  end
+  let* _ = u64_add counter 1 in Return counter
 
 (** [betree_main::betree::fresh_node_id] *)
 let betree_fresh_node_id_back (counter : u64) : result u64 = u64_add counter 1
@@ -57,18 +50,14 @@ let betree_node_id_counter_new_fwd : result betree_node_id_counter_t =
 (** [betree_main::betree::NodeIdCounter::{0}::fresh_id] *)
 let betree_node_id_counter_fresh_id_fwd
   (self : betree_node_id_counter_t) : result u64 =
-  begin match u64_add self.betree_node_id_counter_next_node_id 1 with
-  | Fail e -> Fail e
-  | Return _ -> Return self.betree_node_id_counter_next_node_id
-  end
+  let* _ = u64_add self.betree_node_id_counter_next_node_id 1 in
+  Return self.betree_node_id_counter_next_node_id
 
 (** [betree_main::betree::NodeIdCounter::{0}::fresh_id] *)
 let betree_node_id_counter_fresh_id_back
   (self : betree_node_id_counter_t) : result betree_node_id_counter_t =
-  begin match u64_add self.betree_node_id_counter_next_node_id 1 with
-  | Fail e -> Fail e
-  | Return i -> Return (Mkbetree_node_id_counter_t i)
-  end
+  let* i = u64_add self.betree_node_id_counter_next_node_id 1 in
+  Return (Mkbetree_node_id_counter_t i)
 
 (** [core::num::u64::{10}::MAX] *)
 let core_num_u64_max_body : result u64 = Return 18446744073709551615
@@ -86,11 +75,8 @@ let betree_upsert_update_fwd
   | Some prev0 ->
     begin match st with
     | BetreeUpsertFunStateAdd v ->
-      begin match u64_sub core_num_u64_max_c prev0 with
-      | Fail e -> Fail e
-      | Return margin ->
-        if margin >= v then u64_add prev0 v else Return core_num_u64_max_c
-      end
+      let* margin = u64_sub core_num_u64_max_c prev0 in
+      if margin >= v then u64_add prev0 v else Return core_num_u64_max_c
     | BetreeUpsertFunStateSub v ->
       if prev0 >= v then u64_sub prev0 v else Return 0
     end
@@ -102,11 +88,7 @@ let rec betree_list_len_fwd
   Tot (result u64) (decreases (betree_list_len_decreases t self))
   =
   begin match self with
-  | BetreeListCons x tl ->
-    begin match betree_list_len_fwd t tl with
-    | Fail e -> Fail e
-    | Return i -> u64_add 1 i
-    end
+  | BetreeListCons x tl -> let* i = betree_list_len_fwd t tl in u64_add 1 i
   | BetreeListNil -> Return 0
   end
 
@@ -121,17 +103,11 @@ let rec betree_list_split_at_fwd
   else
     begin match self with
     | BetreeListCons hd tl ->
-      begin match u64_sub n 1 with
-      | Fail e -> Fail e
-      | Return i ->
-        begin match betree_list_split_at_fwd t tl i with
-        | Fail e -> Fail e
-        | Return p ->
-          let (ls0, ls1) = p in
-          let l = ls0 in
-          Return (BetreeListCons hd l, ls1)
-        end
-      end
+      let* i = u64_sub n 1 in
+      let* p = betree_list_split_at_fwd t tl i in
+      let (ls0, ls1) = p in
+      let l = ls0 in
+      Return (BetreeListCons hd l, ls1)
     | BetreeListNil -> Fail Failure
     end
 
@@ -185,14 +161,11 @@ let rec betree_list_partition_at_pivot_fwd
     let (i, x) = hd in
     if i >= pivot
     then Return (BetreeListNil, BetreeListCons (i, x) tl)
-    else
-      begin match betree_list_partition_at_pivot_fwd t tl pivot with
-      | Fail e -> Fail e
-      | Return p ->
-        let (ls0, ls1) = p in
-        let l = ls0 in
-        Return (BetreeListCons (i, x) l, ls1)
-      end
+    else begin
+      let* p = betree_list_partition_at_pivot_fwd t tl pivot in
+      let (ls0, ls1) = p in
+      let l = ls0 in
+      Return (BetreeListCons (i, x) l, ls1) end
   | BetreeListNil -> Return (BetreeListNil, BetreeListNil)
   end
 
@@ -203,44 +176,22 @@ let betree_leaf_split_fwd
   (st : state) :
   result (state & betree_internal_t)
   =
-  begin match
+  let* p =
     betree_list_split_at_fwd (u64 & u64) content
-      params.betree_params_split_size with
-  | Fail e -> Fail e
-  | Return p ->
-    let (content0, content1) = p in
-    begin match betree_list_hd_fwd (u64 & u64) content1 with
-    | Fail e -> Fail e
-    | Return p0 ->
-      let (pivot, _) = p0 in
-      begin match betree_node_id_counter_fresh_id_fwd node_id_cnt with
-      | Fail e -> Fail e
-      | Return id0 ->
-        begin match betree_node_id_counter_fresh_id_back node_id_cnt with
-        | Fail e -> Fail e
-        | Return node_id_cnt0 ->
-          begin match betree_node_id_counter_fresh_id_fwd node_id_cnt0 with
-          | Fail e -> Fail e
-          | Return id1 ->
-            begin match betree_store_leaf_node_fwd id0 content0 st with
-            | Fail e -> Fail e
-            | Return (st0, _) ->
-              begin match betree_store_leaf_node_fwd id1 content1 st0 with
-              | Fail e -> Fail e
-              | Return (st1, _) ->
-                let n = BetreeNodeLeaf (Mkbetree_leaf_t id0
-                  params.betree_params_split_size) in
-                let n0 = BetreeNodeLeaf (Mkbetree_leaf_t id1
-                  params.betree_params_split_size) in
-                Return (st1, Mkbetree_internal_t self.betree_leaf_id pivot n
-                  n0)
-              end
-            end
-          end
-        end
-      end
-    end
-  end
+      params.betree_params_split_size in
+  let (content0, content1) = p in
+  let* p0 = betree_list_hd_fwd (u64 & u64) content1 in
+  let (pivot, _) = p0 in
+  let* id0 = betree_node_id_counter_fresh_id_fwd node_id_cnt in
+  let* node_id_cnt0 = betree_node_id_counter_fresh_id_back node_id_cnt in
+  let* id1 = betree_node_id_counter_fresh_id_fwd node_id_cnt0 in
+  let* (st0, _) = betree_store_leaf_node_fwd id0 content0 st in
+  let* (st1, _) = betree_store_leaf_node_fwd id1 content1 st0 in
+  let n = BetreeNodeLeaf (Mkbetree_leaf_t id0 params.betree_params_split_size)
+    in
+  let n0 = BetreeNodeLeaf (Mkbetree_leaf_t id1 params.betree_params_split_size)
+    in
+  Return (st1, Mkbetree_internal_t self.betree_leaf_id pivot n n0)
 
 (** [betree_main::betree::Leaf::{3}::split] *)
 let betree_leaf_split_back
@@ -249,37 +200,17 @@ let betree_leaf_split_back
   (st : state) :
   result betree_node_id_counter_t
   =
-  begin match
+  let* p =
     betree_list_split_at_fwd (u64 & u64) content
-      params.betree_params_split_size with
-  | Fail e -> Fail e
-  | Return p ->
-    let (content0, content1) = p in
-    begin match betree_list_hd_fwd (u64 & u64) content1 with
-    | Fail e -> Fail e
-    | Return _ ->
-      begin match betree_node_id_counter_fresh_id_fwd node_id_cnt with
-      | Fail e -> Fail e
-      | Return id0 ->
-        begin match betree_node_id_counter_fresh_id_back node_id_cnt with
-        | Fail e -> Fail e
-        | Return node_id_cnt0 ->
-          begin match betree_node_id_counter_fresh_id_fwd node_id_cnt0 with
-          | Fail e -> Fail e
-          | Return id1 ->
-            begin match betree_store_leaf_node_fwd id0 content0 st with
-            | Fail e -> Fail e
-            | Return (st0, _) ->
-              begin match betree_store_leaf_node_fwd id1 content1 st0 with
-              | Fail e -> Fail e
-              | Return _ -> betree_node_id_counter_fresh_id_back node_id_cnt0
-              end
-            end
-          end
-        end
-      end
-    end
-  end
+      params.betree_params_split_size in
+  let (content0, content1) = p in
+  let* _ = betree_list_hd_fwd (u64 & u64) content1 in
+  let* id0 = betree_node_id_counter_fresh_id_fwd node_id_cnt in
+  let* node_id_cnt0 = betree_node_id_counter_fresh_id_back node_id_cnt in
+  let* id1 = betree_node_id_counter_fresh_id_fwd node_id_cnt0 in
+  let* (st0, _) = betree_store_leaf_node_fwd id0 content0 st in
+  let* _ = betree_store_leaf_node_fwd id1 content1 st0 in
+  betree_node_id_counter_fresh_id_back node_id_cnt0
 
 (** [betree_main::betree::Node::{5}::lookup_in_bindings] *)
 let rec betree_node_lookup_in_bindings_fwd
@@ -326,12 +257,10 @@ let rec betree_node_lookup_first_message_for_key_back
     let (i, m) = x in
     if i >= key
     then Return ret
-    else
-      begin match
-        betree_node_lookup_first_message_for_key_back key next_msgs ret with
-      | Fail e -> Fail e
-      | Return next_msgs0 -> Return (BetreeListCons (i, m) next_msgs0)
-      end
+    else begin
+      let* next_msgs0 =
+        betree_node_lookup_first_message_for_key_back key next_msgs ret in
+      Return (BetreeListCons (i, m) next_msgs0) end
   | BetreeListNil -> Return ret
   end
 
@@ -342,43 +271,25 @@ let rec betree_node_apply_upserts_fwd
   Tot (result (state & u64))
   (decreases (betree_node_apply_upserts_decreases msgs prev key st))
   =
-  begin match betree_list_head_has_key_fwd betree_message_t msgs key with
-  | Fail e -> Fail e
-  | Return b ->
-    if b
-    then
-      begin match betree_list_pop_front_fwd (u64 & betree_message_t) msgs with
-      | Fail e -> Fail e
-      | Return msg ->
-        let (_, m) = msg in
-        begin match m with
-        | BetreeMessageInsert i -> Fail Failure
-        | BetreeMessageDelete -> Fail Failure
-        | BetreeMessageUpsert s ->
-          begin match betree_upsert_update_fwd prev s with
-          | Fail e -> Fail e
-          | Return v ->
-            begin match
-              betree_list_pop_front_back (u64 & betree_message_t) msgs with
-            | Fail e -> Fail e
-            | Return msgs0 ->
-              betree_node_apply_upserts_fwd msgs0 (Some v) key st
-            end
-          end
-        end
-      end
-    else
-      begin match core_option_option_unwrap_fwd u64 prev st with
-      | Fail e -> Fail e
-      | Return (st0, v) ->
-        begin match
-          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs (key,
-            BetreeMessageInsert v) with
-        | Fail e -> Fail e
-        | Return _ -> Return (st0, v)
-        end
-      end
-  end
+  let* b = betree_list_head_has_key_fwd betree_message_t msgs key in
+  if b
+  then begin
+    let* msg = betree_list_pop_front_fwd (u64 & betree_message_t) msgs in
+    let (_, m) = msg in
+    begin match m with
+    | BetreeMessageInsert i -> Fail Failure
+    | BetreeMessageDelete -> Fail Failure
+    | BetreeMessageUpsert s ->
+      let* v = betree_upsert_update_fwd prev s in
+      let* msgs0 = betree_list_pop_front_back (u64 & betree_message_t) msgs in
+      betree_node_apply_upserts_fwd msgs0 (Some v) key st
+    end end
+  else begin
+    let* (st0, v) = core_option_option_unwrap_fwd u64 prev st in
+    let* _ =
+      betree_list_push_front_fwd_back (u64 & betree_message_t) msgs (key,
+        BetreeMessageInsert v) in
+    Return (st0, v) end
 
 (** [betree_main::betree::Node::{5}::apply_upserts] *)
 let rec betree_node_apply_upserts_back
@@ -387,39 +298,23 @@ let rec betree_node_apply_upserts_back
   Tot (result (betree_list_t (u64 & betree_message_t)))
   (decreases (betree_node_apply_upserts_decreases msgs prev key st))
   =
-  begin match betree_list_head_has_key_fwd betree_message_t msgs key with
-  | Fail e -> Fail e
-  | Return b ->
-    if b
-    then
-      begin match betree_list_pop_front_fwd (u64 & betree_message_t) msgs with
-      | Fail e -> Fail e
-      | Return msg ->
-        let (_, m) = msg in
-        begin match m with
-        | BetreeMessageInsert i -> Fail Failure
-        | BetreeMessageDelete -> Fail Failure
-        | BetreeMessageUpsert s ->
-          begin match betree_upsert_update_fwd prev s with
-          | Fail e -> Fail e
-          | Return v ->
-            begin match
-              betree_list_pop_front_back (u64 & betree_message_t) msgs with
-            | Fail e -> Fail e
-            | Return msgs0 ->
-              betree_node_apply_upserts_back msgs0 (Some v) key st
-            end
-          end
-        end
-      end
-    else
-      begin match core_option_option_unwrap_fwd u64 prev st with
-      | Fail e -> Fail e
-      | Return (_, v) ->
-        betree_list_push_front_fwd_back (u64 & betree_message_t) msgs (key,
-          BetreeMessageInsert v)
-      end
-  end
+  let* b = betree_list_head_has_key_fwd betree_message_t msgs key in
+  if b
+  then begin
+    let* msg = betree_list_pop_front_fwd (u64 & betree_message_t) msgs in
+    let (_, m) = msg in
+    begin match m with
+    | BetreeMessageInsert i -> Fail Failure
+    | BetreeMessageDelete -> Fail Failure
+    | BetreeMessageUpsert s ->
+      let* v = betree_upsert_update_fwd prev s in
+      let* msgs0 = betree_list_pop_front_back (u64 & betree_message_t) msgs in
+      betree_node_apply_upserts_back msgs0 (Some v) key st
+    end end
+  else begin
+    let* (_, v) = core_option_option_unwrap_fwd u64 prev st in
+    betree_list_push_front_fwd_back (u64 & betree_message_t) msgs (key,
+      BetreeMessageInsert v) end
 
 (** [betree_main::betree::Node::{5}::lookup] *)
 let rec betree_node_lookup_fwd
@@ -429,103 +324,59 @@ let rec betree_node_lookup_fwd
   =
   begin match self with
   | BetreeNodeInternal node ->
-    begin match betree_load_internal_node_fwd node.betree_internal_id st with
-    | Fail e -> Fail e
-    | Return (st0, msgs) ->
-      begin match betree_node_lookup_first_message_for_key_fwd key msgs with
-      | Fail e -> Fail e
-      | Return pending ->
-        begin match pending with
-        | BetreeListCons p l ->
-          let (k, msg) = p in
-          if k <> key
-          then
-            begin match betree_internal_lookup_in_children_fwd node key st0
-              with
-            | Fail e -> Fail e
-            | Return (st1, opt) ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, msg) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (st1, opt)
-              end
-            end
-          else
-            begin match msg with
-            | BetreeMessageInsert v ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, BetreeMessageInsert v) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (st0, Some v)
-              end
-            | BetreeMessageDelete ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, BetreeMessageDelete) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (st0, None)
-              end
-            | BetreeMessageUpsert ufs ->
-              begin match betree_internal_lookup_in_children_fwd node key st0
-                with
-              | Fail e -> Fail e
-              | Return (st1, v) ->
-                begin match
-                  betree_node_apply_upserts_fwd (BetreeListCons (k,
-                    BetreeMessageUpsert ufs) l) v key st1 with
-                | Fail e -> Fail e
-                | Return (st2, v0) ->
-                  begin match
-                    betree_internal_lookup_in_children_back node key st0 with
-                  | Fail e -> Fail e
-                  | Return node0 ->
-                    begin match
-                      betree_node_apply_upserts_back (BetreeListCons (k,
-                        BetreeMessageUpsert ufs) l) v key st1 with
-                    | Fail e -> Fail e
-                    | Return pending0 ->
-                      begin match
-                        betree_node_lookup_first_message_for_key_back key msgs
-                          pending0 with
-                      | Fail e -> Fail e
-                      | Return msgs0 ->
-                        begin match
-                          betree_store_internal_node_fwd
-                            node0.betree_internal_id msgs0 st2 with
-                        | Fail e -> Fail e
-                        | Return (st3, _) -> Return (st3, Some v0)
-                        end
-                      end
-                    end
-                  end
-                end
-              end
-            end
-        | BetreeListNil ->
-          begin match betree_internal_lookup_in_children_fwd node key st0 with
-          | Fail e -> Fail e
-          | Return (st1, opt) ->
-            begin match
-              betree_node_lookup_first_message_for_key_back key msgs
-                BetreeListNil with
-            | Fail e -> Fail e
-            | Return _ -> Return (st1, opt)
-            end
-          end
+    let* (st0, msgs) = betree_load_internal_node_fwd node.betree_internal_id st
+      in
+    let* pending = betree_node_lookup_first_message_for_key_fwd key msgs in
+    begin match pending with
+    | BetreeListCons p l ->
+      let (k, msg) = p in
+      if k <> key
+      then begin
+        let* (st1, opt) = betree_internal_lookup_in_children_fwd node key st0
+          in
+        let* _ =
+          betree_node_lookup_first_message_for_key_back key msgs
+            (BetreeListCons (k, msg) l) in
+        Return (st1, opt) end
+      else
+        begin match msg with
+        | BetreeMessageInsert v ->
+          let* _ =
+            betree_node_lookup_first_message_for_key_back key msgs
+              (BetreeListCons (k, BetreeMessageInsert v) l) in
+          Return (st0, Some v)
+        | BetreeMessageDelete ->
+          let* _ =
+            betree_node_lookup_first_message_for_key_back key msgs
+              (BetreeListCons (k, BetreeMessageDelete) l) in
+          Return (st0, None)
+        | BetreeMessageUpsert ufs ->
+          let* (st1, v) = betree_internal_lookup_in_children_fwd node key st0
+            in
+          let* (st2, v0) =
+            betree_node_apply_upserts_fwd (BetreeListCons (k,
+              BetreeMessageUpsert ufs) l) v key st1 in
+          let* node0 = betree_internal_lookup_in_children_back node key st0 in
+          let* pending0 =
+            betree_node_apply_upserts_back (BetreeListCons (k,
+              BetreeMessageUpsert ufs) l) v key st1 in
+          let* msgs0 =
+            betree_node_lookup_first_message_for_key_back key msgs pending0 in
+          let* (st3, _) =
+            betree_store_internal_node_fwd node0.betree_internal_id msgs0 st2
+            in
+          Return (st3, Some v0)
         end
-      end
+    | BetreeListNil ->
+      let* (st1, opt) = betree_internal_lookup_in_children_fwd node key st0 in
+      let* _ =
+        betree_node_lookup_first_message_for_key_back key msgs BetreeListNil in
+      Return (st1, opt)
     end
   | BetreeNodeLeaf node ->
-    begin match betree_load_leaf_node_fwd node.betree_leaf_id st with
-    | Fail e -> Fail e
-    | Return (st0, bindings) ->
-      begin match betree_node_lookup_in_bindings_fwd key bindings with
-      | Fail e -> Fail e
-      | Return opt -> Return (st0, opt)
-      end
-    end
+    let* (st0, bindings) = betree_load_leaf_node_fwd node.betree_leaf_id st in
+    let* opt = betree_node_lookup_in_bindings_fwd key bindings in
+    Return (st0, opt)
   end
 
 (** [betree_main::betree::Node::{5}::lookup] *)
@@ -536,104 +387,58 @@ and betree_node_lookup_back
   =
   begin match self with
   | BetreeNodeInternal node ->
-    begin match betree_load_internal_node_fwd node.betree_internal_id st with
-    | Fail e -> Fail e
-    | Return (st0, msgs) ->
-      begin match betree_node_lookup_first_message_for_key_fwd key msgs with
-      | Fail e -> Fail e
-      | Return pending ->
-        begin match pending with
-        | BetreeListCons p l ->
-          let (k, msg) = p in
-          if k <> key
-          then
-            begin match
-              betree_node_lookup_first_message_for_key_back key msgs
-                (BetreeListCons (k, msg) l) with
-            | Fail e -> Fail e
-            | Return _ ->
-              begin match betree_internal_lookup_in_children_back node key st0
-                with
-              | Fail e -> Fail e
-              | Return node0 -> Return (BetreeNodeInternal node0)
-              end
-            end
-          else
-            begin match msg with
-            | BetreeMessageInsert v ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, BetreeMessageInsert v) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (BetreeNodeInternal node)
-              end
-            | BetreeMessageDelete ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, BetreeMessageDelete) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (BetreeNodeInternal node)
-              end
-            | BetreeMessageUpsert ufs ->
-              begin match betree_internal_lookup_in_children_fwd node key st0
-                with
-              | Fail e -> Fail e
-              | Return (st1, v) ->
-                begin match
-                  betree_node_apply_upserts_fwd (BetreeListCons (k,
-                    BetreeMessageUpsert ufs) l) v key st1 with
-                | Fail e -> Fail e
-                | Return (st2, _) ->
-                  begin match
-                    betree_internal_lookup_in_children_back node key st0 with
-                  | Fail e -> Fail e
-                  | Return node0 ->
-                    begin match
-                      betree_node_apply_upserts_back (BetreeListCons (k,
-                        BetreeMessageUpsert ufs) l) v key st1 with
-                    | Fail e -> Fail e
-                    | Return pending0 ->
-                      begin match
-                        betree_node_lookup_first_message_for_key_back key msgs
-                          pending0 with
-                      | Fail e -> Fail e
-                      | Return msgs0 ->
-                        begin match
-                          betree_store_internal_node_fwd
-                            node0.betree_internal_id msgs0 st2 with
-                        | Fail e -> Fail e
-                        | Return _ -> Return (BetreeNodeInternal node0)
-                        end
-                      end
-                    end
-                  end
-                end
-              end
-            end
-        | BetreeListNil ->
-          begin match
+    let* (st0, msgs) = betree_load_internal_node_fwd node.betree_internal_id st
+      in
+    let* pending = betree_node_lookup_first_message_for_key_fwd key msgs in
+    begin match pending with
+    | BetreeListCons p l ->
+      let (k, msg) = p in
+      if k <> key
+      then begin
+        let* _ =
+          betree_node_lookup_first_message_for_key_back key msgs
+            (BetreeListCons (k, msg) l) in
+        let* node0 = betree_internal_lookup_in_children_back node key st0 in
+        Return (BetreeNodeInternal node0) end
+      else
+        begin match msg with
+        | BetreeMessageInsert v ->
+          let* _ =
+            betree_node_lookup_first_message_for_key_back key msgs
+              (BetreeListCons (k, BetreeMessageInsert v) l) in
+          Return (BetreeNodeInternal node)
+        | BetreeMessageDelete ->
+          let* _ =
             betree_node_lookup_first_message_for_key_back key msgs
-              BetreeListNil with
-          | Fail e -> Fail e
-          | Return _ ->
-            begin match betree_internal_lookup_in_children_back node key st0
-              with
-            | Fail e -> Fail e
-            | Return node0 -> Return (BetreeNodeInternal node0)
-            end
-          end
+              (BetreeListCons (k, BetreeMessageDelete) l) in
+          Return (BetreeNodeInternal node)
+        | BetreeMessageUpsert ufs ->
+          let* (st1, v) = betree_internal_lookup_in_children_fwd node key st0
+            in
+          let* (st2, _) =
+            betree_node_apply_upserts_fwd (BetreeListCons (k,
+              BetreeMessageUpsert ufs) l) v key st1 in
+          let* node0 = betree_internal_lookup_in_children_back node key st0 in
+          let* pending0 =
+            betree_node_apply_upserts_back (BetreeListCons (k,
+              BetreeMessageUpsert ufs) l) v key st1 in
+          let* msgs0 =
+            betree_node_lookup_first_message_for_key_back key msgs pending0 in
+          let* _ =
+            betree_store_internal_node_fwd node0.betree_internal_id msgs0 st2
+            in
+          Return (BetreeNodeInternal node0)
         end
-      end
+    | BetreeListNil ->
+      let* _ =
+        betree_node_lookup_first_message_for_key_back key msgs BetreeListNil in
+      let* node0 = betree_internal_lookup_in_children_back node key st0 in
+      Return (BetreeNodeInternal node0)
     end
   | BetreeNodeLeaf node ->
-    begin match betree_load_leaf_node_fwd node.betree_leaf_id st with
-    | Fail e -> Fail e
-    | Return (_, bindings) ->
-      begin match betree_node_lookup_in_bindings_fwd key bindings with
-      | Fail e -> Fail e
-      | Return _ -> Return (BetreeNodeLeaf node)
-      end
-    end
+    let* (_, bindings) = betree_load_leaf_node_fwd node.betree_leaf_id st in
+    let* _ = betree_node_lookup_in_bindings_fwd key bindings in
+    Return (BetreeNodeLeaf node)
   end
 
 (** [betree_main::betree::Internal::{4}::lookup_in_children] *)
@@ -653,20 +458,14 @@ and betree_internal_lookup_in_children_back
   (decreases (betree_internal_lookup_in_children_decreases self key st))
   =
   if key < self.betree_internal_pivot
-  then
-    begin match betree_node_lookup_back self.betree_internal_left key st with
-    | Fail e -> Fail e
-    | Return n ->
-      Return (Mkbetree_internal_t self.betree_internal_id
-        self.betree_internal_pivot n self.betree_internal_right)
-    end
-  else
-    begin match betree_node_lookup_back self.betree_internal_right key st with
-    | Fail e -> Fail e
-    | Return n ->
-      Return (Mkbetree_internal_t self.betree_internal_id
-        self.betree_internal_pivot self.betree_internal_left n)
-    end
+  then begin
+    let* n = betree_node_lookup_back self.betree_internal_left key st in
+    Return (Mkbetree_internal_t self.betree_internal_id
+      self.betree_internal_pivot n self.betree_internal_right) end
+  else begin
+    let* n = betree_node_lookup_back self.betree_internal_right key st in
+    Return (Mkbetree_internal_t self.betree_internal_id
+      self.betree_internal_pivot self.betree_internal_left n) end
 
 (** [betree_main::betree::Node::{5}::lookup_mut_in_bindings] *)
 let rec betree_node_lookup_mut_in_bindings_fwd
@@ -695,11 +494,9 @@ let rec betree_node_lookup_mut_in_bindings_back
     let (i, i0) = hd in
     if i >= key
     then Return ret
-    else
-      begin match betree_node_lookup_mut_in_bindings_back key tl ret with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (BetreeListCons (i, i0) tl0)
-      end
+    else begin
+      let* tl0 = betree_node_lookup_mut_in_bindings_back key tl ret in
+      Return (BetreeListCons (i, i0) tl0) end
   | BetreeListNil -> Return ret
   end
 
@@ -709,82 +506,42 @@ let betree_node_apply_to_leaf_fwd_back
   (new_msg : betree_message_t) :
   result (betree_list_t (u64 & u64))
   =
-  begin match betree_node_lookup_mut_in_bindings_fwd key bindings with
-  | Fail e -> Fail e
-  | Return bindings0 ->
-    begin match betree_list_head_has_key_fwd u64 bindings0 key with
-    | Fail e -> Fail e
-    | Return b ->
-      if b
-      then
-        begin match betree_list_pop_front_fwd (u64 & u64) bindings0 with
-        | Fail e -> Fail e
-        | Return hd ->
-          begin match new_msg with
-          | BetreeMessageInsert v ->
-            begin match betree_list_pop_front_back (u64 & u64) bindings0 with
-            | Fail e -> Fail e
-            | Return bindings1 ->
-              begin match
-                betree_list_push_front_fwd_back (u64 & u64) bindings1 (key, v)
-                with
-              | Fail e -> Fail e
-              | Return bindings2 ->
-                betree_node_lookup_mut_in_bindings_back key bindings bindings2
-              end
-            end
-          | BetreeMessageDelete ->
-            begin match betree_list_pop_front_back (u64 & u64) bindings0 with
-            | Fail e -> Fail e
-            | Return bindings1 ->
-              betree_node_lookup_mut_in_bindings_back key bindings bindings1
-            end
-          | BetreeMessageUpsert s ->
-            let (_, i) = hd in
-            begin match betree_upsert_update_fwd (Some i) s with
-            | Fail e -> Fail e
-            | Return v ->
-              begin match betree_list_pop_front_back (u64 & u64) bindings0 with
-              | Fail e -> Fail e
-              | Return bindings1 ->
-                begin match
-                  betree_list_push_front_fwd_back (u64 & u64) bindings1 (key,
-                    v) with
-                | Fail e -> Fail e
-                | Return bindings2 ->
-                  betree_node_lookup_mut_in_bindings_back key bindings
-                    bindings2
-                end
-              end
-            end
-          end
-        end
-      else
-        begin match new_msg with
-        | BetreeMessageInsert v ->
-          begin match
-            betree_list_push_front_fwd_back (u64 & u64) bindings0 (key, v) with
-          | Fail e -> Fail e
-          | Return bindings1 ->
-            betree_node_lookup_mut_in_bindings_back key bindings bindings1
-          end
-        | BetreeMessageDelete ->
-          betree_node_lookup_mut_in_bindings_back key bindings bindings0
-        | BetreeMessageUpsert s ->
-          begin match betree_upsert_update_fwd None s with
-          | Fail e -> Fail e
-          | Return v ->
-            begin match
-              betree_list_push_front_fwd_back (u64 & u64) bindings0 (key, v)
-              with
-            | Fail e -> Fail e
-            | Return bindings1 ->
-              betree_node_lookup_mut_in_bindings_back key bindings bindings1
-            end
-          end
-        end
+  let* bindings0 = betree_node_lookup_mut_in_bindings_fwd key bindings in
+  let* b = betree_list_head_has_key_fwd u64 bindings0 key in
+  if b
+  then begin
+    let* hd = betree_list_pop_front_fwd (u64 & u64) bindings0 in
+    begin match new_msg with
+    | BetreeMessageInsert v ->
+      let* bindings1 = betree_list_pop_front_back (u64 & u64) bindings0 in
+      let* bindings2 =
+        betree_list_push_front_fwd_back (u64 & u64) bindings1 (key, v) in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings2
+    | BetreeMessageDelete ->
+      let* bindings1 = betree_list_pop_front_back (u64 & u64) bindings0 in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings1
+    | BetreeMessageUpsert s ->
+      let (_, i) = hd in
+      let* v = betree_upsert_update_fwd (Some i) s in
+      let* bindings1 = betree_list_pop_front_back (u64 & u64) bindings0 in
+      let* bindings2 =
+        betree_list_push_front_fwd_back (u64 & u64) bindings1 (key, v) in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings2
+    end end
+  else
+    begin match new_msg with
+    | BetreeMessageInsert v ->
+      let* bindings1 =
+        betree_list_push_front_fwd_back (u64 & u64) bindings0 (key, v) in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings1
+    | BetreeMessageDelete ->
+      betree_node_lookup_mut_in_bindings_back key bindings bindings0
+    | BetreeMessageUpsert s ->
+      let* v = betree_upsert_update_fwd None s in
+      let* bindings1 =
+        betree_list_push_front_fwd_back (u64 & u64) bindings0 (key, v) in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings1
     end
-  end
 
 (** [betree_main::betree::Node::{5}::apply_messages_to_leaf] *)
 let rec betree_node_apply_messages_to_leaf_fwd_back
@@ -796,11 +553,8 @@ let rec betree_node_apply_messages_to_leaf_fwd_back
   begin match new_msgs with
   | BetreeListCons new_msg new_msgs_tl ->
     let (i, m) = new_msg in
-    begin match betree_node_apply_to_leaf_fwd_back bindings i m with
-    | Fail e -> Fail e
-    | Return bindings0 ->
-      betree_node_apply_messages_to_leaf_fwd_back bindings0 new_msgs_tl
-    end
+    let* bindings0 = betree_node_apply_to_leaf_fwd_back bindings i m in
+    betree_node_apply_messages_to_leaf_fwd_back bindings0 new_msgs_tl
   | BetreeListNil -> Return bindings
   end
 
@@ -814,13 +568,11 @@ let rec betree_node_filter_messages_for_key_fwd_back
   | BetreeListCons p l ->
     let (k, m) = p in
     if k = key
-    then
-      begin match
+    then begin
+      let* msgs0 =
         betree_list_pop_front_back (u64 & betree_message_t) (BetreeListCons (k,
-          m) l) with
-      | Fail e -> Fail e
-      | Return msgs0 -> betree_node_filter_messages_for_key_fwd_back key msgs0
-      end
+          m) l) in
+      betree_node_filter_messages_for_key_fwd_back key msgs0 end
     else Return (BetreeListCons (k, m) l)
   | BetreeListNil -> Return BetreeListNil
   end
@@ -851,12 +603,10 @@ let rec betree_node_lookup_first_message_after_key_back
   | BetreeListCons p next_msgs ->
     let (k, m) = p in
     if k = key
-    then
-      begin match
-        betree_node_lookup_first_message_after_key_back key next_msgs ret with
-      | Fail e -> Fail e
-      | Return next_msgs0 -> Return (BetreeListCons (k, m) next_msgs0)
-      end
+    then begin
+      let* next_msgs0 =
+        betree_node_lookup_first_message_after_key_back key next_msgs ret in
+      Return (BetreeListCons (k, m) next_msgs0) end
     else Return ret
   | BetreeListNil -> Return ret
   end
@@ -867,118 +617,59 @@ let betree_node_apply_to_internal_fwd_back
   (new_msg : betree_message_t) :
   result (betree_list_t (u64 & betree_message_t))
   =
-  begin match betree_node_lookup_first_message_for_key_fwd key msgs with
-  | Fail e -> Fail e
-  | Return msgs0 ->
-    begin match betree_list_head_has_key_fwd betree_message_t msgs0 key with
-    | Fail e -> Fail e
-    | Return b ->
-      if b
-      then
-        begin match new_msg with
-        | BetreeMessageInsert i ->
-          begin match betree_node_filter_messages_for_key_fwd_back key msgs0
-            with
-          | Fail e -> Fail e
-          | Return msgs1 ->
-            begin match
-              betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1
-                (key, BetreeMessageInsert i) with
-            | Fail e -> Fail e
-            | Return msgs2 ->
-              betree_node_lookup_first_message_for_key_back key msgs msgs2
-            end
-          end
-        | BetreeMessageDelete ->
-          begin match betree_node_filter_messages_for_key_fwd_back key msgs0
-            with
-          | Fail e -> Fail e
-          | Return msgs1 ->
-            begin match
-              betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1
-                (key, BetreeMessageDelete) with
-            | Fail e -> Fail e
-            | Return msgs2 ->
-              betree_node_lookup_first_message_for_key_back key msgs msgs2
-            end
-          end
-        | BetreeMessageUpsert s ->
-          begin match betree_list_hd_fwd (u64 & betree_message_t) msgs0 with
-          | Fail e -> Fail e
-          | Return p ->
-            let (_, m) = p in
-            begin match m with
-            | BetreeMessageInsert prev ->
-              begin match betree_upsert_update_fwd (Some prev) s with
-              | Fail e -> Fail e
-              | Return v ->
-                begin match
-                  betree_list_pop_front_back (u64 & betree_message_t) msgs0
-                  with
-                | Fail e -> Fail e
-                | Return msgs1 ->
-                  begin match
-                    betree_list_push_front_fwd_back (u64 & betree_message_t)
-                      msgs1 (key, BetreeMessageInsert v) with
-                  | Fail e -> Fail e
-                  | Return msgs2 ->
-                    betree_node_lookup_first_message_for_key_back key msgs
-                      msgs2
-                  end
-                end
-              end
-            | BetreeMessageDelete ->
-              begin match betree_upsert_update_fwd None s with
-              | Fail e -> Fail e
-              | Return v ->
-                begin match
-                  betree_list_pop_front_back (u64 & betree_message_t) msgs0
-                  with
-                | Fail e -> Fail e
-                | Return msgs1 ->
-                  begin match
-                    betree_list_push_front_fwd_back (u64 & betree_message_t)
-                      msgs1 (key, BetreeMessageInsert v) with
-                  | Fail e -> Fail e
-                  | Return msgs2 ->
-                    betree_node_lookup_first_message_for_key_back key msgs
-                      msgs2
-                  end
-                end
-              end
-            | BetreeMessageUpsert ufs ->
-              begin match
-                betree_node_lookup_first_message_after_key_fwd key msgs0 with
-              | Fail e -> Fail e
-              | Return msgs1 ->
-                begin match
-                  betree_list_push_front_fwd_back (u64 & betree_message_t)
-                    msgs1 (key, BetreeMessageUpsert s) with
-                | Fail e -> Fail e
-                | Return msgs2 ->
-                  begin match
-                    betree_node_lookup_first_message_after_key_back key msgs0
-                      msgs2 with
-                  | Fail e -> Fail e
-                  | Return msgs3 ->
-                    betree_node_lookup_first_message_for_key_back key msgs
-                      msgs3
-                  end
-                end
-              end
-            end
-          end
-        end
-      else
-        begin match
-          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs0 (key,
-            new_msg) with
-        | Fail e -> Fail e
-        | Return msgs1 ->
-          betree_node_lookup_first_message_for_key_back key msgs msgs1
-        end
+  let* msgs0 = betree_node_lookup_first_message_for_key_fwd key msgs in
+  let* b = betree_list_head_has_key_fwd betree_message_t msgs0 key in
+  if b
+  then
+    begin match new_msg with
+    | BetreeMessageInsert i ->
+      let* msgs1 = betree_node_filter_messages_for_key_fwd_back key msgs0 in
+      let* msgs2 =
+        betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+          BetreeMessageInsert i) in
+      betree_node_lookup_first_message_for_key_back key msgs msgs2
+    | BetreeMessageDelete ->
+      let* msgs1 = betree_node_filter_messages_for_key_fwd_back key msgs0 in
+      let* msgs2 =
+        betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+          BetreeMessageDelete) in
+      betree_node_lookup_first_message_for_key_back key msgs msgs2
+    | BetreeMessageUpsert s ->
+      let* p = betree_list_hd_fwd (u64 & betree_message_t) msgs0 in
+      let (_, m) = p in
+      begin match m with
+      | BetreeMessageInsert prev ->
+        let* v = betree_upsert_update_fwd (Some prev) s in
+        let* msgs1 = betree_list_pop_front_back (u64 & betree_message_t) msgs0
+          in
+        let* msgs2 =
+          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+            BetreeMessageInsert v) in
+        betree_node_lookup_first_message_for_key_back key msgs msgs2
+      | BetreeMessageDelete ->
+        let* v = betree_upsert_update_fwd None s in
+        let* msgs1 = betree_list_pop_front_back (u64 & betree_message_t) msgs0
+          in
+        let* msgs2 =
+          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+            BetreeMessageInsert v) in
+        betree_node_lookup_first_message_for_key_back key msgs msgs2
+      | BetreeMessageUpsert ufs ->
+        let* msgs1 = betree_node_lookup_first_message_after_key_fwd key msgs0
+          in
+        let* msgs2 =
+          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+            BetreeMessageUpsert s) in
+        let* msgs3 =
+          betree_node_lookup_first_message_after_key_back key msgs0 msgs2 in
+        betree_node_lookup_first_message_for_key_back key msgs msgs3
+      end
     end
-  end
+  else begin
+    let* msgs1 =
+      betree_list_push_front_fwd_back (u64 & betree_message_t) msgs0 (key,
+        new_msg) in
+    betree_node_lookup_first_message_for_key_back key msgs msgs1 end
 
 (** [betree_main::betree::Node::{5}::apply_messages_to_internal] *)
 let rec betree_node_apply_messages_to_internal_fwd_back
@@ -990,11 +681,8 @@ let rec betree_node_apply_messages_to_internal_fwd_back
   begin match new_msgs with
   | BetreeListCons new_msg new_msgs_tl ->
     let (i, m) = new_msg in
-    begin match betree_node_apply_to_internal_fwd_back msgs i m with
-    | Fail e -> Fail e
-    | Return msgs0 ->
-      betree_node_apply_messages_to_internal_fwd_back msgs0 new_msgs_tl
-    end
+    let* msgs0 = betree_node_apply_to_internal_fwd_back msgs i m in
+    betree_node_apply_messages_to_internal_fwd_back msgs0 new_msgs_tl
   | BetreeListNil -> Return msgs
   end
 
@@ -1009,83 +697,40 @@ let rec betree_node_apply_messages_fwd
   =
   begin match self with
   | BetreeNodeInternal node ->
-    begin match betree_load_internal_node_fwd node.betree_internal_id st with
-    | Fail e -> Fail e
-    | Return (st0, content) ->
-      begin match betree_node_apply_messages_to_internal_fwd_back content msgs
-        with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & betree_message_t) content0 with
-        | Fail e -> Fail e
-        | Return num_msgs ->
-          if num_msgs >= params.betree_params_min_flush_size
-          then
-            begin match
-              betree_internal_flush_fwd node params node_id_cnt content0 st0
-              with
-            | Fail e -> Fail e
-            | Return (st1, content1) ->
-              begin match
-                betree_internal_flush_back node params node_id_cnt content0 st0
-                with
-              | Fail e -> Fail e
-              | Return (node0, _) ->
-                begin match
-                  betree_store_internal_node_fwd node0.betree_internal_id
-                    content1 st1 with
-                | Fail e -> Fail e
-                | Return (st2, _) -> Return (st2, ())
-                end
-              end
-            end
-          else
-            begin match
-              betree_store_internal_node_fwd node.betree_internal_id content0
-                st0 with
-            | Fail e -> Fail e
-            | Return (st1, _) -> Return (st1, ())
-            end
-        end
-      end
-    end
+    let* (st0, content) =
+      betree_load_internal_node_fwd node.betree_internal_id st in
+    let* content0 =
+      betree_node_apply_messages_to_internal_fwd_back content msgs in
+    let* num_msgs = betree_list_len_fwd (u64 & betree_message_t) content0 in
+    if num_msgs >= params.betree_params_min_flush_size
+    then begin
+      let* (st1, content1) =
+        betree_internal_flush_fwd node params node_id_cnt content0 st0 in
+      let* (node0, _) =
+        betree_internal_flush_back node params node_id_cnt content0 st0 in
+      let* (st2, _) =
+        betree_store_internal_node_fwd node0.betree_internal_id content1 st1 in
+      Return (st2, ()) end
+    else begin
+      let* (st1, _) =
+        betree_store_internal_node_fwd node.betree_internal_id content0 st0 in
+      Return (st1, ()) end
   | BetreeNodeLeaf node ->
-    begin match betree_load_leaf_node_fwd node.betree_leaf_id st with
-    | Fail e -> Fail e
-    | Return (st0, content) ->
-      begin match betree_node_apply_messages_to_leaf_fwd_back content msgs with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & u64) content0 with
-        | Fail e -> Fail e
-        | Return len ->
-          begin match u64_mul 2 params.betree_params_split_size with
-          | Fail e -> Fail e
-          | Return i ->
-            if len >= i
-            then
-              begin match
-                betree_leaf_split_fwd node content0 params node_id_cnt st0 with
-              | Fail e -> Fail e
-              | Return (st1, _) ->
-                begin match
-                  betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil
-                    st1 with
-                | Fail e -> Fail e
-                | Return (st2, _) -> Return (st2, ())
-                end
-              end
-            else
-              begin match
-                betree_store_leaf_node_fwd node.betree_leaf_id content0 st0
-                with
-              | Fail e -> Fail e
-              | Return (st1, _) -> Return (st1, ())
-              end
-          end
-        end
-      end
-    end
+    let* (st0, content) = betree_load_leaf_node_fwd node.betree_leaf_id st in
+    let* content0 = betree_node_apply_messages_to_leaf_fwd_back content msgs in
+    let* len = betree_list_len_fwd (u64 & u64) content0 in
+    let* i = u64_mul 2 params.betree_params_split_size in
+    if len >= i
+    then begin
+      let* (st1, _) =
+        betree_leaf_split_fwd node content0 params node_id_cnt st0 in
+      let* (st2, _) =
+        betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil st1 in
+      Return (st2, ()) end
+    else begin
+      let* (st1, _) =
+        betree_store_leaf_node_fwd node.betree_leaf_id content0 st0 in
+      Return (st1, ()) end
   end
 
 (** [betree_main::betree::Node::{5}::apply_messages] *)
@@ -1099,92 +744,42 @@ and betree_node_apply_messages_back
   =
   begin match self with
   | BetreeNodeInternal node ->
-    begin match betree_load_internal_node_fwd node.betree_internal_id st with
-    | Fail e -> Fail e
-    | Return (st0, content) ->
-      begin match betree_node_apply_messages_to_internal_fwd_back content msgs
-        with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & betree_message_t) content0 with
-        | Fail e -> Fail e
-        | Return num_msgs ->
-          if num_msgs >= params.betree_params_min_flush_size
-          then
-            begin match
-              betree_internal_flush_fwd node params node_id_cnt content0 st0
-              with
-            | Fail e -> Fail e
-            | Return (st1, content1) ->
-              begin match
-                betree_internal_flush_back node params node_id_cnt content0 st0
-                with
-              | Fail e -> Fail e
-              | Return (node0, node_id_cnt0) ->
-                begin match
-                  betree_store_internal_node_fwd node0.betree_internal_id
-                    content1 st1 with
-                | Fail e -> Fail e
-                | Return _ -> Return (BetreeNodeInternal node0, node_id_cnt0)
-                end
-              end
-            end
-          else
-            begin match
-              betree_store_internal_node_fwd node.betree_internal_id content0
-                st0 with
-            | Fail e -> Fail e
-            | Return _ -> Return (BetreeNodeInternal node, node_id_cnt)
-            end
-        end
-      end
-    end
+    let* (st0, content) =
+      betree_load_internal_node_fwd node.betree_internal_id st in
+    let* content0 =
+      betree_node_apply_messages_to_internal_fwd_back content msgs in
+    let* num_msgs = betree_list_len_fwd (u64 & betree_message_t) content0 in
+    if num_msgs >= params.betree_params_min_flush_size
+    then begin
+      let* (st1, content1) =
+        betree_internal_flush_fwd node params node_id_cnt content0 st0 in
+      let* (node0, node_id_cnt0) =
+        betree_internal_flush_back node params node_id_cnt content0 st0 in
+      let* _ =
+        betree_store_internal_node_fwd node0.betree_internal_id content1 st1 in
+      Return (BetreeNodeInternal node0, node_id_cnt0) end
+    else begin
+      let* _ =
+        betree_store_internal_node_fwd node.betree_internal_id content0 st0 in
+      Return (BetreeNodeInternal node, node_id_cnt) end
   | BetreeNodeLeaf node ->
-    begin match betree_load_leaf_node_fwd node.betree_leaf_id st with
-    | Fail e -> Fail e
-    | Return (st0, content) ->
-      begin match betree_node_apply_messages_to_leaf_fwd_back content msgs with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & u64) content0 with
-        | Fail e -> Fail e
-        | Return len ->
-          begin match u64_mul 2 params.betree_params_split_size with
-          | Fail e -> Fail e
-          | Return i ->
-            if len >= i
-            then
-              begin match
-                betree_leaf_split_fwd node content0 params node_id_cnt st0 with
-              | Fail e -> Fail e
-              | Return (st1, new_node) ->
-                begin match
-                  betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil
-                    st1 with
-                | Fail e -> Fail e
-                | Return _ ->
-                  begin match
-                    betree_leaf_split_back node content0 params node_id_cnt st0
-                    with
-                  | Fail e -> Fail e
-                  | Return node_id_cnt0 ->
-                    Return (BetreeNodeInternal new_node, node_id_cnt0)
-                  end
-                end
-              end
-            else
-              begin match
-                betree_store_leaf_node_fwd node.betree_leaf_id content0 st0
-                with
-              | Fail e -> Fail e
-              | Return _ ->
-                Return (BetreeNodeLeaf (Mkbetree_leaf_t node.betree_leaf_id
-                  len), node_id_cnt)
-              end
-          end
-        end
-      end
-    end
+    let* (st0, content) = betree_load_leaf_node_fwd node.betree_leaf_id st in
+    let* content0 = betree_node_apply_messages_to_leaf_fwd_back content msgs in
+    let* len = betree_list_len_fwd (u64 & u64) content0 in
+    let* i = u64_mul 2 params.betree_params_split_size in
+    if len >= i
+    then begin
+      let* (st1, new_node) =
+        betree_leaf_split_fwd node content0 params node_id_cnt st0 in
+      let* _ = betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil st1
+        in
+      let* node_id_cnt0 =
+        betree_leaf_split_back node content0 params node_id_cnt st0 in
+      Return (BetreeNodeInternal new_node, node_id_cnt0) end
+    else begin
+      let* _ = betree_store_leaf_node_fwd node.betree_leaf_id content0 st0 in
+      Return (BetreeNodeLeaf (Mkbetree_leaf_t node.betree_leaf_id len),
+        node_id_cnt) end
   end
 
 (** [betree_main::betree::Internal::{4}::flush] *)
@@ -1196,64 +791,38 @@ and betree_internal_flush_fwd
   (decreases (
     betree_internal_flush_decreases self params node_id_cnt content st))
   =
-  begin match
+  let* p =
     betree_list_partition_at_pivot_fwd betree_message_t content
-      self.betree_internal_pivot with
-  | Fail e -> Fail e
-  | Return p ->
-    let (msgs_left, msgs_right) = p in
-    begin match betree_list_len_fwd (u64 & betree_message_t) msgs_left with
-    | Fail e -> Fail e
-    | Return len_left ->
-      if len_left >= params.betree_params_min_flush_size
-      then
-        begin match
-          betree_node_apply_messages_fwd self.betree_internal_left params
-            node_id_cnt msgs_left st with
-        | Fail e -> Fail e
-        | Return (st0, _) ->
-          begin match
-            betree_node_apply_messages_back self.betree_internal_left params
-              node_id_cnt msgs_left st with
-          | Fail e -> Fail e
-          | Return (_, node_id_cnt0) ->
-            begin match betree_list_len_fwd (u64 & betree_message_t) msgs_right
-              with
-            | Fail e -> Fail e
-            | Return len_right ->
-              if len_right >= params.betree_params_min_flush_size
-              then
-                begin match
-                  betree_node_apply_messages_fwd self.betree_internal_right
-                    params node_id_cnt0 msgs_right st0 with
-                | Fail e -> Fail e
-                | Return (st1, _) ->
-                  begin match
-                    betree_node_apply_messages_back self.betree_internal_right
-                      params node_id_cnt0 msgs_right st0 with
-                  | Fail e -> Fail e
-                  | Return _ -> Return (st1, BetreeListNil)
-                  end
-                end
-              else Return (st0, msgs_right)
-            end
-          end
-        end
-      else
-        begin match
-          betree_node_apply_messages_fwd self.betree_internal_right params
-            node_id_cnt msgs_right st with
-        | Fail e -> Fail e
-        | Return (st0, _) ->
-          begin match
-            betree_node_apply_messages_back self.betree_internal_right params
-              node_id_cnt msgs_right st with
-          | Fail e -> Fail e
-          | Return _ -> Return (st0, msgs_left)
-          end
-        end
-    end
-  end
+      self.betree_internal_pivot in
+  let (msgs_left, msgs_right) = p in
+  let* len_left = betree_list_len_fwd (u64 & betree_message_t) msgs_left in
+  if len_left >= params.betree_params_min_flush_size
+  then begin
+    let* (st0, _) =
+      betree_node_apply_messages_fwd self.betree_internal_left params
+        node_id_cnt msgs_left st in
+    let* (_, node_id_cnt0) =
+      betree_node_apply_messages_back self.betree_internal_left params
+        node_id_cnt msgs_left st in
+    let* len_right = betree_list_len_fwd (u64 & betree_message_t) msgs_right in
+    if len_right >= params.betree_params_min_flush_size
+    then begin
+      let* (st1, _) =
+        betree_node_apply_messages_fwd self.betree_internal_right params
+          node_id_cnt0 msgs_right st0 in
+      let* _ =
+        betree_node_apply_messages_back self.betree_internal_right params
+          node_id_cnt0 msgs_right st0 in
+      Return (st1, BetreeListNil) end
+    else Return (st0, msgs_right) end
+  else begin
+    let* (st0, _) =
+      betree_node_apply_messages_fwd self.betree_internal_right params
+        node_id_cnt msgs_right st in
+    let* _ =
+      betree_node_apply_messages_back self.betree_internal_right params
+        node_id_cnt msgs_right st in
+    Return (st0, msgs_left) end
 
 (** [betree_main::betree::Internal::{4}::flush] *)
 and betree_internal_flush_back
@@ -1264,60 +833,37 @@ and betree_internal_flush_back
   (decreases (
     betree_internal_flush_decreases self params node_id_cnt content st))
   =
-  begin match
+  let* p =
     betree_list_partition_at_pivot_fwd betree_message_t content
-      self.betree_internal_pivot with
-  | Fail e -> Fail e
-  | Return p ->
-    let (msgs_left, msgs_right) = p in
-    begin match betree_list_len_fwd (u64 & betree_message_t) msgs_left with
-    | Fail e -> Fail e
-    | Return len_left ->
-      if len_left >= params.betree_params_min_flush_size
-      then
-        begin match
-          betree_node_apply_messages_fwd self.betree_internal_left params
-            node_id_cnt msgs_left st with
-        | Fail e -> Fail e
-        | Return (st0, _) ->
-          begin match
-            betree_node_apply_messages_back self.betree_internal_left params
-              node_id_cnt msgs_left st with
-          | Fail e -> Fail e
-          | Return (n, node_id_cnt0) ->
-            begin match betree_list_len_fwd (u64 & betree_message_t) msgs_right
-              with
-            | Fail e -> Fail e
-            | Return len_right ->
-              if len_right >= params.betree_params_min_flush_size
-              then
-                begin match
-                  betree_node_apply_messages_back self.betree_internal_right
-                    params node_id_cnt0 msgs_right st0 with
-                | Fail e -> Fail e
-                | Return (n0, node_id_cnt1) ->
-                  Return (Mkbetree_internal_t self.betree_internal_id
-                    self.betree_internal_pivot n n0, node_id_cnt1)
-                end
-              else
-                Return (Mkbetree_internal_t self.betree_internal_id
-                  self.betree_internal_pivot n self.betree_internal_right,
-                  node_id_cnt0)
-            end
-          end
-        end
-      else
-        begin match
-          betree_node_apply_messages_back self.betree_internal_right params
-            node_id_cnt msgs_right st with
-        | Fail e -> Fail e
-        | Return (n, node_id_cnt0) ->
-          Return (Mkbetree_internal_t self.betree_internal_id
-            self.betree_internal_pivot self.betree_internal_left n,
-            node_id_cnt0)
-        end
+      self.betree_internal_pivot in
+  let (msgs_left, msgs_right) = p in
+  let* len_left = betree_list_len_fwd (u64 & betree_message_t) msgs_left in
+  if len_left >= params.betree_params_min_flush_size
+  then begin
+    let* (st0, _) =
+      betree_node_apply_messages_fwd self.betree_internal_left params
+        node_id_cnt msgs_left st in
+    let* (n, node_id_cnt0) =
+      betree_node_apply_messages_back self.betree_internal_left params
+        node_id_cnt msgs_left st in
+    let* len_right = betree_list_len_fwd (u64 & betree_message_t) msgs_right in
+    if len_right >= params.betree_params_min_flush_size
+    then begin
+      let* (n0, node_id_cnt1) =
+        betree_node_apply_messages_back self.betree_internal_right params
+          node_id_cnt0 msgs_right st0 in
+      Return (Mkbetree_internal_t self.betree_internal_id
+        self.betree_internal_pivot n n0, node_id_cnt1) end
+    else
+      Return (Mkbetree_internal_t self.betree_internal_id
+        self.betree_internal_pivot n self.betree_internal_right, node_id_cnt0)
     end
-  end
+  else begin
+    let* (n, node_id_cnt0) =
+      betree_node_apply_messages_back self.betree_internal_right params
+        node_id_cnt msgs_right st in
+    Return (Mkbetree_internal_t self.betree_internal_id
+      self.betree_internal_pivot self.betree_internal_left n, node_id_cnt0) end
 
 (** [betree_main::betree::Node::{5}::apply] *)
 let betree_node_apply_fwd
@@ -1327,18 +873,13 @@ let betree_node_apply_fwd
   result (state & unit)
   =
   let l = BetreeListNil in
-  begin match
+  let* (st0, _) =
     betree_node_apply_messages_fwd self params node_id_cnt (BetreeListCons
-      (key, new_msg) l) st with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match
-      betree_node_apply_messages_back self params node_id_cnt (BetreeListCons
-        (key, new_msg) l) st with
-    | Fail e -> Fail e
-    | Return _ -> Return (st0, ())
-    end
-  end
+      (key, new_msg) l) st in
+  let* _ =
+    betree_node_apply_messages_back self params node_id_cnt (BetreeListCons
+      (key, new_msg) l) st in
+  Return (st0, ())
 
 (** [betree_main::betree::Node::{5}::apply] *)
 let betree_node_apply_back
@@ -1356,72 +897,45 @@ let betree_be_tree_new_fwd
   (min_flush_size : u64) (split_size : u64) (st : state) :
   result (state & betree_be_tree_t)
   =
-  begin match betree_node_id_counter_new_fwd with
-  | Fail e -> Fail e
-  | Return node_id_cnt ->
-    begin match betree_node_id_counter_fresh_id_fwd node_id_cnt with
-    | Fail e -> Fail e
-    | Return id ->
-      begin match betree_store_leaf_node_fwd id BetreeListNil st with
-      | Fail e -> Fail e
-      | Return (st0, _) ->
-        begin match betree_node_id_counter_fresh_id_back node_id_cnt with
-        | Fail e -> Fail e
-        | Return node_id_cnt0 ->
-          Return (st0, Mkbetree_be_tree_t (Mkbetree_params_t min_flush_size
-            split_size) node_id_cnt0 (BetreeNodeLeaf (Mkbetree_leaf_t id 0)))
-        end
-      end
-    end
-  end
+  let* node_id_cnt = betree_node_id_counter_new_fwd in
+  let* id = betree_node_id_counter_fresh_id_fwd node_id_cnt in
+  let* (st0, _) = betree_store_leaf_node_fwd id BetreeListNil st in
+  let* node_id_cnt0 = betree_node_id_counter_fresh_id_back node_id_cnt in
+  Return (st0, Mkbetree_be_tree_t (Mkbetree_params_t min_flush_size split_size)
+    node_id_cnt0 (BetreeNodeLeaf (Mkbetree_leaf_t id 0)))
 
 (** [betree_main::betree::BeTree::{6}::apply] *)
 let betree_be_tree_apply_fwd
   (self : betree_be_tree_t) (key : u64) (msg : betree_message_t) (st : state) :
   result (state & unit)
   =
-  begin match
+  let* (st0, _) =
     betree_node_apply_fwd self.betree_be_tree_root self.betree_be_tree_params
-      self.betree_be_tree_node_id_cnt key msg st with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match
-      betree_node_apply_back self.betree_be_tree_root
-        self.betree_be_tree_params self.betree_be_tree_node_id_cnt key msg st
-      with
-    | Fail e -> Fail e
-    | Return _ -> Return (st0, ())
-    end
-  end
+      self.betree_be_tree_node_id_cnt key msg st in
+  let* _ =
+    betree_node_apply_back self.betree_be_tree_root self.betree_be_tree_params
+      self.betree_be_tree_node_id_cnt key msg st in
+  Return (st0, ())
 
 (** [betree_main::betree::BeTree::{6}::apply] *)
 let betree_be_tree_apply_back
   (self : betree_be_tree_t) (key : u64) (msg : betree_message_t) (st : state) :
   result betree_be_tree_t
   =
-  begin match
+  let* (n, nic) =
     betree_node_apply_back self.betree_be_tree_root self.betree_be_tree_params
-      self.betree_be_tree_node_id_cnt key msg st with
-  | Fail e -> Fail e
-  | Return (n, nic) ->
-    Return (Mkbetree_be_tree_t self.betree_be_tree_params nic n)
-  end
+      self.betree_be_tree_node_id_cnt key msg st in
+  Return (Mkbetree_be_tree_t self.betree_be_tree_params nic n)
 
 (** [betree_main::betree::BeTree::{6}::insert] *)
 let betree_be_tree_insert_fwd
   (self : betree_be_tree_t) (key : u64) (value : u64) (st : state) :
   result (state & unit)
   =
-  begin match betree_be_tree_apply_fwd self key (BetreeMessageInsert value) st
-    with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match
-      betree_be_tree_apply_back self key (BetreeMessageInsert value) st with
-    | Fail e -> Fail e
-    | Return _ -> Return (st0, ())
-    end
-  end
+  let* (st0, _) =
+    betree_be_tree_apply_fwd self key (BetreeMessageInsert value) st in
+  let* _ = betree_be_tree_apply_back self key (BetreeMessageInsert value) st in
+  Return (st0, ())
 
 (** [betree_main::betree::BeTree::{6}::insert] *)
 let betree_be_tree_insert_back
@@ -1433,14 +947,9 @@ let betree_be_tree_insert_back
 (** [betree_main::betree::BeTree::{6}::delete] *)
 let betree_be_tree_delete_fwd
   (self : betree_be_tree_t) (key : u64) (st : state) : result (state & unit) =
-  begin match betree_be_tree_apply_fwd self key BetreeMessageDelete st with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match betree_be_tree_apply_back self key BetreeMessageDelete st with
-    | Fail e -> Fail e
-    | Return _ -> Return (st0, ())
-    end
-  end
+  let* (st0, _) = betree_be_tree_apply_fwd self key BetreeMessageDelete st in
+  let* _ = betree_be_tree_apply_back self key BetreeMessageDelete st in
+  Return (st0, ())
 
 (** [betree_main::betree::BeTree::{6}::delete] *)
 let betree_be_tree_delete_back
@@ -1455,16 +964,10 @@ let betree_be_tree_upsert_fwd
   (st : state) :
   result (state & unit)
   =
-  begin match betree_be_tree_apply_fwd self key (BetreeMessageUpsert upd) st
-    with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match betree_be_tree_apply_back self key (BetreeMessageUpsert upd) st
-      with
-    | Fail e -> Fail e
-    | Return _ -> Return (st0, ())
-    end
-  end
+  let* (st0, _) =
+    betree_be_tree_apply_fwd self key (BetreeMessageUpsert upd) st in
+  let* _ = betree_be_tree_apply_back self key (BetreeMessageUpsert upd) st in
+  Return (st0, ())
 
 (** [betree_main::betree::BeTree::{6}::upsert] *)
 let betree_be_tree_upsert_back
@@ -1486,12 +989,9 @@ let betree_be_tree_lookup_back
   (self : betree_be_tree_t) (key : u64) (st : state) :
   result betree_be_tree_t
   =
-  begin match betree_node_lookup_back self.betree_be_tree_root key st with
-  | Fail e -> Fail e
-  | Return n ->
-    Return (Mkbetree_be_tree_t self.betree_be_tree_params
-      self.betree_be_tree_node_id_cnt n)
-  end
+  let* n = betree_node_lookup_back self.betree_be_tree_root key st in
+  Return (Mkbetree_be_tree_t self.betree_be_tree_params
+    self.betree_be_tree_node_id_cnt n)
 
 (** [betree_main::main] *)
 let main_fwd : result unit = Return ()
diff --git a/tests/fstar/betree/Primitives.fst b/tests/fstar/betree/Primitives.fst
index bf0f9078..98a696b6 100644
--- a/tests/fstar/betree/Primitives.fst
+++ b/tests/fstar/betree/Primitives.fst
@@ -35,7 +35,9 @@ unfold let return (#a : Type0) (x : a) : result a = Return x
 // let* x = y in
 // ...
 // ```
-unfold let (let*) (#a #b : Type0) (m: result a) (f: a -> result b) : result b =
+unfold let (let*) (#a #b : Type0) (m: result a)
+  (f: (x:a) -> Pure (result b) (requires (m == Return x)) (ensures fun _ -> True)) :
+  result b =
   match m with
   | Return x -> f x
   | Fail e   -> Fail e
diff --git a/tests/fstar/betree_back_stateful/BetreeMain.Funs.fst b/tests/fstar/betree_back_stateful/BetreeMain.Funs.fst
index 2a336271..201778df 100644
--- a/tests/fstar/betree_back_stateful/BetreeMain.Funs.fst
+++ b/tests/fstar/betree_back_stateful/BetreeMain.Funs.fst
@@ -20,10 +20,8 @@ let betree_store_internal_node_fwd
   (id : u64) (content : betree_list_t (u64 & betree_message_t)) (st : state) :
   result (state & unit)
   =
-  begin match betree_utils_store_internal_node_fwd id content st with
-  | Fail e -> Fail e
-  | Return (st0, _) -> Return (st0, ())
-  end
+  let* (st0, _) = betree_utils_store_internal_node_fwd id content st in
+  Return (st0, ())
 
 (** [betree_main::betree::load_leaf_node] *)
 let betree_load_leaf_node_fwd
@@ -35,17 +33,12 @@ let betree_store_leaf_node_fwd
   (id : u64) (content : betree_list_t (u64 & u64)) (st : state) :
   result (state & unit)
   =
-  begin match betree_utils_store_leaf_node_fwd id content st with
-  | Fail e -> Fail e
-  | Return (st0, _) -> Return (st0, ())
-  end
+  let* (st0, _) = betree_utils_store_leaf_node_fwd id content st in
+  Return (st0, ())
 
 (** [betree_main::betree::fresh_node_id] *)
 let betree_fresh_node_id_fwd (counter : u64) : result u64 =
-  begin match u64_add counter 1 with
-  | Fail e -> Fail e
-  | Return _ -> Return counter
-  end
+  let* _ = u64_add counter 1 in Return counter
 
 (** [betree_main::betree::fresh_node_id] *)
 let betree_fresh_node_id_back (counter : u64) : result u64 = u64_add counter 1
@@ -57,18 +50,14 @@ let betree_node_id_counter_new_fwd : result betree_node_id_counter_t =
 (** [betree_main::betree::NodeIdCounter::{0}::fresh_id] *)
 let betree_node_id_counter_fresh_id_fwd
   (self : betree_node_id_counter_t) : result u64 =
-  begin match u64_add self.betree_node_id_counter_next_node_id 1 with
-  | Fail e -> Fail e
-  | Return _ -> Return self.betree_node_id_counter_next_node_id
-  end
+  let* _ = u64_add self.betree_node_id_counter_next_node_id 1 in
+  Return self.betree_node_id_counter_next_node_id
 
 (** [betree_main::betree::NodeIdCounter::{0}::fresh_id] *)
 let betree_node_id_counter_fresh_id_back
   (self : betree_node_id_counter_t) : result betree_node_id_counter_t =
-  begin match u64_add self.betree_node_id_counter_next_node_id 1 with
-  | Fail e -> Fail e
-  | Return i -> Return (Mkbetree_node_id_counter_t i)
-  end
+  let* i = u64_add self.betree_node_id_counter_next_node_id 1 in
+  Return (Mkbetree_node_id_counter_t i)
 
 (** [core::num::u64::{10}::MAX] *)
 let core_num_u64_max_body : result u64 = Return 18446744073709551615
@@ -86,11 +75,8 @@ let betree_upsert_update_fwd
   | Some prev0 ->
     begin match st with
     | BetreeUpsertFunStateAdd v ->
-      begin match u64_sub core_num_u64_max_c prev0 with
-      | Fail e -> Fail e
-      | Return margin ->
-        if margin >= v then u64_add prev0 v else Return core_num_u64_max_c
-      end
+      let* margin = u64_sub core_num_u64_max_c prev0 in
+      if margin >= v then u64_add prev0 v else Return core_num_u64_max_c
     | BetreeUpsertFunStateSub v ->
       if prev0 >= v then u64_sub prev0 v else Return 0
     end
@@ -102,11 +88,7 @@ let rec betree_list_len_fwd
   Tot (result u64) (decreases (betree_list_len_decreases t self))
   =
   begin match self with
-  | BetreeListCons x tl ->
-    begin match betree_list_len_fwd t tl with
-    | Fail e -> Fail e
-    | Return i -> u64_add 1 i
-    end
+  | BetreeListCons x tl -> let* i = betree_list_len_fwd t tl in u64_add 1 i
   | BetreeListNil -> Return 0
   end
 
@@ -121,17 +103,11 @@ let rec betree_list_split_at_fwd
   else
     begin match self with
     | BetreeListCons hd tl ->
-      begin match u64_sub n 1 with
-      | Fail e -> Fail e
-      | Return i ->
-        begin match betree_list_split_at_fwd t tl i with
-        | Fail e -> Fail e
-        | Return p ->
-          let (ls0, ls1) = p in
-          let l = ls0 in
-          Return (BetreeListCons hd l, ls1)
-        end
-      end
+      let* i = u64_sub n 1 in
+      let* p = betree_list_split_at_fwd t tl i in
+      let (ls0, ls1) = p in
+      let l = ls0 in
+      Return (BetreeListCons hd l, ls1)
     | BetreeListNil -> Fail Failure
     end
 
@@ -185,14 +161,11 @@ let rec betree_list_partition_at_pivot_fwd
     let (i, x) = hd in
     if i >= pivot
     then Return (BetreeListNil, BetreeListCons (i, x) tl)
-    else
-      begin match betree_list_partition_at_pivot_fwd t tl pivot with
-      | Fail e -> Fail e
-      | Return p ->
-        let (ls0, ls1) = p in
-        let l = ls0 in
-        Return (BetreeListCons (i, x) l, ls1)
-      end
+    else begin
+      let* p = betree_list_partition_at_pivot_fwd t tl pivot in
+      let (ls0, ls1) = p in
+      let l = ls0 in
+      Return (BetreeListCons (i, x) l, ls1) end
   | BetreeListNil -> Return (BetreeListNil, BetreeListNil)
   end
 
@@ -203,44 +176,22 @@ let betree_leaf_split_fwd
   (st : state) :
   result (state & betree_internal_t)
   =
-  begin match
+  let* p =
     betree_list_split_at_fwd (u64 & u64) content
-      params.betree_params_split_size with
-  | Fail e -> Fail e
-  | Return p ->
-    let (content0, content1) = p in
-    begin match betree_list_hd_fwd (u64 & u64) content1 with
-    | Fail e -> Fail e
-    | Return p0 ->
-      let (pivot, _) = p0 in
-      begin match betree_node_id_counter_fresh_id_fwd node_id_cnt with
-      | Fail e -> Fail e
-      | Return id0 ->
-        begin match betree_node_id_counter_fresh_id_back node_id_cnt with
-        | Fail e -> Fail e
-        | Return node_id_cnt0 ->
-          begin match betree_node_id_counter_fresh_id_fwd node_id_cnt0 with
-          | Fail e -> Fail e
-          | Return id1 ->
-            begin match betree_store_leaf_node_fwd id0 content0 st with
-            | Fail e -> Fail e
-            | Return (st0, _) ->
-              begin match betree_store_leaf_node_fwd id1 content1 st0 with
-              | Fail e -> Fail e
-              | Return (st1, _) ->
-                let n = BetreeNodeLeaf (Mkbetree_leaf_t id0
-                  params.betree_params_split_size) in
-                let n0 = BetreeNodeLeaf (Mkbetree_leaf_t id1
-                  params.betree_params_split_size) in
-                Return (st1, Mkbetree_internal_t self.betree_leaf_id pivot n
-                  n0)
-              end
-            end
-          end
-        end
-      end
-    end
-  end
+      params.betree_params_split_size in
+  let (content0, content1) = p in
+  let* p0 = betree_list_hd_fwd (u64 & u64) content1 in
+  let (pivot, _) = p0 in
+  let* id0 = betree_node_id_counter_fresh_id_fwd node_id_cnt in
+  let* node_id_cnt0 = betree_node_id_counter_fresh_id_back node_id_cnt in
+  let* id1 = betree_node_id_counter_fresh_id_fwd node_id_cnt0 in
+  let* (st0, _) = betree_store_leaf_node_fwd id0 content0 st in
+  let* (st1, _) = betree_store_leaf_node_fwd id1 content1 st0 in
+  let n = BetreeNodeLeaf (Mkbetree_leaf_t id0 params.betree_params_split_size)
+    in
+  let n0 = BetreeNodeLeaf (Mkbetree_leaf_t id1 params.betree_params_split_size)
+    in
+  Return (st1, Mkbetree_internal_t self.betree_leaf_id pivot n n0)
 
 (** [betree_main::betree::Leaf::{3}::split] *)
 let betree_leaf_split_back0
@@ -249,37 +200,17 @@ let betree_leaf_split_back0
   (st : state) (st0 : state) :
   result (state & unit)
   =
-  begin match
+  let* p =
     betree_list_split_at_fwd (u64 & u64) content
-      params.betree_params_split_size with
-  | Fail e -> Fail e
-  | Return p ->
-    let (content0, content1) = p in
-    begin match betree_list_hd_fwd (u64 & u64) content1 with
-    | Fail e -> Fail e
-    | Return _ ->
-      begin match betree_node_id_counter_fresh_id_fwd node_id_cnt with
-      | Fail e -> Fail e
-      | Return id0 ->
-        begin match betree_node_id_counter_fresh_id_back node_id_cnt with
-        | Fail e -> Fail e
-        | Return node_id_cnt0 ->
-          begin match betree_node_id_counter_fresh_id_fwd node_id_cnt0 with
-          | Fail e -> Fail e
-          | Return id1 ->
-            begin match betree_store_leaf_node_fwd id0 content0 st with
-            | Fail e -> Fail e
-            | Return (st1, _) ->
-              begin match betree_store_leaf_node_fwd id1 content1 st1 with
-              | Fail e -> Fail e
-              | Return _ -> Return (st0, ())
-              end
-            end
-          end
-        end
-      end
-    end
-  end
+      params.betree_params_split_size in
+  let (content0, content1) = p in
+  let* _ = betree_list_hd_fwd (u64 & u64) content1 in
+  let* id0 = betree_node_id_counter_fresh_id_fwd node_id_cnt in
+  let* node_id_cnt0 = betree_node_id_counter_fresh_id_back node_id_cnt in
+  let* id1 = betree_node_id_counter_fresh_id_fwd node_id_cnt0 in
+  let* (st1, _) = betree_store_leaf_node_fwd id0 content0 st in
+  let* _ = betree_store_leaf_node_fwd id1 content1 st1 in
+  Return (st0, ())
 
 (** [betree_main::betree::Leaf::{3}::split] *)
 let betree_leaf_split_back1
@@ -288,37 +219,17 @@ let betree_leaf_split_back1
   (st : state) (st0 : state) :
   result (state & unit)
   =
-  begin match
+  let* p =
     betree_list_split_at_fwd (u64 & u64) content
-      params.betree_params_split_size with
-  | Fail e -> Fail e
-  | Return p ->
-    let (content0, content1) = p in
-    begin match betree_list_hd_fwd (u64 & u64) content1 with
-    | Fail e -> Fail e
-    | Return _ ->
-      begin match betree_node_id_counter_fresh_id_fwd node_id_cnt with
-      | Fail e -> Fail e
-      | Return id0 ->
-        begin match betree_node_id_counter_fresh_id_back node_id_cnt with
-        | Fail e -> Fail e
-        | Return node_id_cnt0 ->
-          begin match betree_node_id_counter_fresh_id_fwd node_id_cnt0 with
-          | Fail e -> Fail e
-          | Return id1 ->
-            begin match betree_store_leaf_node_fwd id0 content0 st with
-            | Fail e -> Fail e
-            | Return (st1, _) ->
-              begin match betree_store_leaf_node_fwd id1 content1 st1 with
-              | Fail e -> Fail e
-              | Return _ -> Return (st0, ())
-              end
-            end
-          end
-        end
-      end
-    end
-  end
+      params.betree_params_split_size in
+  let (content0, content1) = p in
+  let* _ = betree_list_hd_fwd (u64 & u64) content1 in
+  let* id0 = betree_node_id_counter_fresh_id_fwd node_id_cnt in
+  let* node_id_cnt0 = betree_node_id_counter_fresh_id_back node_id_cnt in
+  let* id1 = betree_node_id_counter_fresh_id_fwd node_id_cnt0 in
+  let* (st1, _) = betree_store_leaf_node_fwd id0 content0 st in
+  let* _ = betree_store_leaf_node_fwd id1 content1 st1 in
+  Return (st0, ())
 
 (** [betree_main::betree::Leaf::{3}::split] *)
 let betree_leaf_split_back2
@@ -327,42 +238,18 @@ let betree_leaf_split_back2
   (st : state) (st0 : state) :
   result (state & betree_node_id_counter_t)
   =
-  begin match
+  let* p =
     betree_list_split_at_fwd (u64 & u64) content
-      params.betree_params_split_size with
-  | Fail e -> Fail e
-  | Return p ->
-    let (content0, content1) = p in
-    begin match betree_list_hd_fwd (u64 & u64) content1 with
-    | Fail e -> Fail e
-    | Return _ ->
-      begin match betree_node_id_counter_fresh_id_fwd node_id_cnt with
-      | Fail e -> Fail e
-      | Return id0 ->
-        begin match betree_node_id_counter_fresh_id_back node_id_cnt with
-        | Fail e -> Fail e
-        | Return node_id_cnt0 ->
-          begin match betree_node_id_counter_fresh_id_fwd node_id_cnt0 with
-          | Fail e -> Fail e
-          | Return id1 ->
-            begin match betree_store_leaf_node_fwd id0 content0 st with
-            | Fail e -> Fail e
-            | Return (st1, _) ->
-              begin match betree_store_leaf_node_fwd id1 content1 st1 with
-              | Fail e -> Fail e
-              | Return _ ->
-                begin match betree_node_id_counter_fresh_id_back node_id_cnt0
-                  with
-                | Fail e -> Fail e
-                | Return node_id_cnt1 -> Return (st0, node_id_cnt1)
-                end
-              end
-            end
-          end
-        end
-      end
-    end
-  end
+      params.betree_params_split_size in
+  let (content0, content1) = p in
+  let* _ = betree_list_hd_fwd (u64 & u64) content1 in
+  let* id0 = betree_node_id_counter_fresh_id_fwd node_id_cnt in
+  let* node_id_cnt0 = betree_node_id_counter_fresh_id_back node_id_cnt in
+  let* id1 = betree_node_id_counter_fresh_id_fwd node_id_cnt0 in
+  let* (st1, _) = betree_store_leaf_node_fwd id0 content0 st in
+  let* _ = betree_store_leaf_node_fwd id1 content1 st1 in
+  let* node_id_cnt1 = betree_node_id_counter_fresh_id_back node_id_cnt0 in
+  Return (st0, node_id_cnt1)
 
 (** [betree_main::betree::Node::{5}::lookup_in_bindings] *)
 let rec betree_node_lookup_in_bindings_fwd
@@ -409,12 +296,10 @@ let rec betree_node_lookup_first_message_for_key_back
     let (i, m) = x in
     if i >= key
     then Return ret
-    else
-      begin match
-        betree_node_lookup_first_message_for_key_back key next_msgs ret with
-      | Fail e -> Fail e
-      | Return next_msgs0 -> Return (BetreeListCons (i, m) next_msgs0)
-      end
+    else begin
+      let* next_msgs0 =
+        betree_node_lookup_first_message_for_key_back key next_msgs ret in
+      Return (BetreeListCons (i, m) next_msgs0) end
   | BetreeListNil -> Return ret
   end
 
@@ -425,43 +310,25 @@ let rec betree_node_apply_upserts_fwd
   Tot (result (state & u64))
   (decreases (betree_node_apply_upserts_decreases msgs prev key st))
   =
-  begin match betree_list_head_has_key_fwd betree_message_t msgs key with
-  | Fail e -> Fail e
-  | Return b ->
-    if b
-    then
-      begin match betree_list_pop_front_fwd (u64 & betree_message_t) msgs with
-      | Fail e -> Fail e
-      | Return msg ->
-        let (_, m) = msg in
-        begin match m with
-        | BetreeMessageInsert i -> Fail Failure
-        | BetreeMessageDelete -> Fail Failure
-        | BetreeMessageUpsert s ->
-          begin match betree_upsert_update_fwd prev s with
-          | Fail e -> Fail e
-          | Return v ->
-            begin match
-              betree_list_pop_front_back (u64 & betree_message_t) msgs with
-            | Fail e -> Fail e
-            | Return msgs0 ->
-              betree_node_apply_upserts_fwd msgs0 (Some v) key st
-            end
-          end
-        end
-      end
-    else
-      begin match core_option_option_unwrap_fwd u64 prev st with
-      | Fail e -> Fail e
-      | Return (st0, v) ->
-        begin match
-          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs (key,
-            BetreeMessageInsert v) with
-        | Fail e -> Fail e
-        | Return _ -> Return (st0, v)
-        end
-      end
-  end
+  let* b = betree_list_head_has_key_fwd betree_message_t msgs key in
+  if b
+  then begin
+    let* msg = betree_list_pop_front_fwd (u64 & betree_message_t) msgs in
+    let (_, m) = msg in
+    begin match m with
+    | BetreeMessageInsert i -> Fail Failure
+    | BetreeMessageDelete -> Fail Failure
+    | BetreeMessageUpsert s ->
+      let* v = betree_upsert_update_fwd prev s in
+      let* msgs0 = betree_list_pop_front_back (u64 & betree_message_t) msgs in
+      betree_node_apply_upserts_fwd msgs0 (Some v) key st
+    end end
+  else begin
+    let* (st0, v) = core_option_option_unwrap_fwd u64 prev st in
+    let* _ =
+      betree_list_push_front_fwd_back (u64 & betree_message_t) msgs (key,
+        BetreeMessageInsert v) in
+    Return (st0, v) end
 
 (** [betree_main::betree::Node::{5}::apply_upserts] *)
 let rec betree_node_apply_upserts_back
@@ -470,43 +337,25 @@ let rec betree_node_apply_upserts_back
   Tot (result (state & (betree_list_t (u64 & betree_message_t))))
   (decreases (betree_node_apply_upserts_decreases msgs prev key st))
   =
-  begin match betree_list_head_has_key_fwd betree_message_t msgs key with
-  | Fail e -> Fail e
-  | Return b ->
-    if b
-    then
-      begin match betree_list_pop_front_fwd (u64 & betree_message_t) msgs with
-      | Fail e -> Fail e
-      | Return msg ->
-        let (_, m) = msg in
-        begin match m with
-        | BetreeMessageInsert i -> Fail Failure
-        | BetreeMessageDelete -> Fail Failure
-        | BetreeMessageUpsert s ->
-          begin match betree_upsert_update_fwd prev s with
-          | Fail e -> Fail e
-          | Return v ->
-            begin match
-              betree_list_pop_front_back (u64 & betree_message_t) msgs with
-            | Fail e -> Fail e
-            | Return msgs0 ->
-              betree_node_apply_upserts_back msgs0 (Some v) key st st0
-            end
-          end
-        end
-      end
-    else
-      begin match core_option_option_unwrap_fwd u64 prev st with
-      | Fail e -> Fail e
-      | Return (_, v) ->
-        begin match
-          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs (key,
-            BetreeMessageInsert v) with
-        | Fail e -> Fail e
-        | Return msgs0 -> Return (st0, msgs0)
-        end
-      end
-  end
+  let* b = betree_list_head_has_key_fwd betree_message_t msgs key in
+  if b
+  then begin
+    let* msg = betree_list_pop_front_fwd (u64 & betree_message_t) msgs in
+    let (_, m) = msg in
+    begin match m with
+    | BetreeMessageInsert i -> Fail Failure
+    | BetreeMessageDelete -> Fail Failure
+    | BetreeMessageUpsert s ->
+      let* v = betree_upsert_update_fwd prev s in
+      let* msgs0 = betree_list_pop_front_back (u64 & betree_message_t) msgs in
+      betree_node_apply_upserts_back msgs0 (Some v) key st st0
+    end end
+  else begin
+    let* (_, v) = core_option_option_unwrap_fwd u64 prev st in
+    let* msgs0 =
+      betree_list_push_front_fwd_back (u64 & betree_message_t) msgs (key,
+        BetreeMessageInsert v) in
+    Return (st0, msgs0) end
 
 (** [betree_main::betree::Node::{5}::lookup] *)
 let rec betree_node_lookup_fwd
@@ -516,104 +365,60 @@ let rec betree_node_lookup_fwd
   =
   begin match self with
   | BetreeNodeInternal node ->
-    begin match betree_load_internal_node_fwd node.betree_internal_id st with
-    | Fail e -> Fail e
-    | Return (st0, msgs) ->
-      begin match betree_node_lookup_first_message_for_key_fwd key msgs with
-      | Fail e -> Fail e
-      | Return pending ->
-        begin match pending with
-        | BetreeListCons p l ->
-          let (k, msg) = p in
-          if k <> key
-          then
-            begin match betree_internal_lookup_in_children_fwd node key st0
-              with
-            | Fail e -> Fail e
-            | Return (st1, opt) ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, msg) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (st1, opt)
-              end
-            end
-          else
-            begin match msg with
-            | BetreeMessageInsert v ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, BetreeMessageInsert v) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (st0, Some v)
-              end
-            | BetreeMessageDelete ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, BetreeMessageDelete) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (st0, None)
-              end
-            | BetreeMessageUpsert ufs ->
-              begin match betree_internal_lookup_in_children_fwd node key st0
-                with
-              | Fail e -> Fail e
-              | Return (st1, v) ->
-                begin match
-                  betree_node_apply_upserts_fwd (BetreeListCons (k,
-                    BetreeMessageUpsert ufs) l) v key st1 with
-                | Fail e -> Fail e
-                | Return (st2, v0) ->
-                  begin match
-                    betree_internal_lookup_in_children_back node key st0 st2
-                    with
-                  | Fail e -> Fail e
-                  | Return (st3, node0) ->
-                    begin match
-                      betree_node_apply_upserts_back (BetreeListCons (k,
-                        BetreeMessageUpsert ufs) l) v key st1 st3 with
-                    | Fail e -> Fail e
-                    | Return (st4, pending0) ->
-                      begin match
-                        betree_node_lookup_first_message_for_key_back key msgs
-                          pending0 with
-                      | Fail e -> Fail e
-                      | Return msgs0 ->
-                        begin match
-                          betree_store_internal_node_fwd
-                            node0.betree_internal_id msgs0 st4 with
-                        | Fail e -> Fail e
-                        | Return (st5, _) -> Return (st5, Some v0)
-                        end
-                      end
-                    end
-                  end
-                end
-              end
-            end
-        | BetreeListNil ->
-          begin match betree_internal_lookup_in_children_fwd node key st0 with
-          | Fail e -> Fail e
-          | Return (st1, opt) ->
-            begin match
-              betree_node_lookup_first_message_for_key_back key msgs
-                BetreeListNil with
-            | Fail e -> Fail e
-            | Return _ -> Return (st1, opt)
-            end
-          end
+    let* (st0, msgs) = betree_load_internal_node_fwd node.betree_internal_id st
+      in
+    let* pending = betree_node_lookup_first_message_for_key_fwd key msgs in
+    begin match pending with
+    | BetreeListCons p l ->
+      let (k, msg) = p in
+      if k <> key
+      then begin
+        let* (st1, opt) = betree_internal_lookup_in_children_fwd node key st0
+          in
+        let* _ =
+          betree_node_lookup_first_message_for_key_back key msgs
+            (BetreeListCons (k, msg) l) in
+        Return (st1, opt) end
+      else
+        begin match msg with
+        | BetreeMessageInsert v ->
+          let* _ =
+            betree_node_lookup_first_message_for_key_back key msgs
+              (BetreeListCons (k, BetreeMessageInsert v) l) in
+          Return (st0, Some v)
+        | BetreeMessageDelete ->
+          let* _ =
+            betree_node_lookup_first_message_for_key_back key msgs
+              (BetreeListCons (k, BetreeMessageDelete) l) in
+          Return (st0, None)
+        | BetreeMessageUpsert ufs ->
+          let* (st1, v) = betree_internal_lookup_in_children_fwd node key st0
+            in
+          let* (st2, v0) =
+            betree_node_apply_upserts_fwd (BetreeListCons (k,
+              BetreeMessageUpsert ufs) l) v key st1 in
+          let* (st3, node0) =
+            betree_internal_lookup_in_children_back node key st0 st2 in
+          let* (st4, pending0) =
+            betree_node_apply_upserts_back (BetreeListCons (k,
+              BetreeMessageUpsert ufs) l) v key st1 st3 in
+          let* msgs0 =
+            betree_node_lookup_first_message_for_key_back key msgs pending0 in
+          let* (st5, _) =
+            betree_store_internal_node_fwd node0.betree_internal_id msgs0 st4
+            in
+          Return (st5, Some v0)
         end
-      end
+    | BetreeListNil ->
+      let* (st1, opt) = betree_internal_lookup_in_children_fwd node key st0 in
+      let* _ =
+        betree_node_lookup_first_message_for_key_back key msgs BetreeListNil in
+      Return (st1, opt)
     end
   | BetreeNodeLeaf node ->
-    begin match betree_load_leaf_node_fwd node.betree_leaf_id st with
-    | Fail e -> Fail e
-    | Return (st0, bindings) ->
-      begin match betree_node_lookup_in_bindings_fwd key bindings with
-      | Fail e -> Fail e
-      | Return opt -> Return (st0, opt)
-      end
-    end
+    let* (st0, bindings) = betree_load_leaf_node_fwd node.betree_leaf_id st in
+    let* opt = betree_node_lookup_in_bindings_fwd key bindings in
+    Return (st0, opt)
   end
 
 (** [betree_main::betree::Node::{5}::lookup] *)
@@ -624,105 +429,61 @@ and betree_node_lookup_back
   =
   begin match self with
   | BetreeNodeInternal node ->
-    begin match betree_load_internal_node_fwd node.betree_internal_id st with
-    | Fail e -> Fail e
-    | Return (st1, msgs) ->
-      begin match betree_node_lookup_first_message_for_key_fwd key msgs with
-      | Fail e -> Fail e
-      | Return pending ->
-        begin match pending with
-        | BetreeListCons p l ->
-          let (k, msg) = p in
-          if k <> key
-          then
-            begin match
-              betree_node_lookup_first_message_for_key_back key msgs
-                (BetreeListCons (k, msg) l) with
-            | Fail e -> Fail e
-            | Return _ ->
-              begin match
-                betree_internal_lookup_in_children_back node key st1 st0 with
-              | Fail e -> Fail e
-              | Return (st2, node0) -> Return (st2, BetreeNodeInternal node0)
-              end
-            end
-          else
-            begin match msg with
-            | BetreeMessageInsert v ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, BetreeMessageInsert v) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (st0, BetreeNodeInternal node)
-              end
-            | BetreeMessageDelete ->
-              begin match
-                betree_node_lookup_first_message_for_key_back key msgs
-                  (BetreeListCons (k, BetreeMessageDelete) l) with
-              | Fail e -> Fail e
-              | Return _ -> Return (st0, BetreeNodeInternal node)
-              end
-            | BetreeMessageUpsert ufs ->
-              begin match betree_internal_lookup_in_children_fwd node key st1
-                with
-              | Fail e -> Fail e
-              | Return (st2, v) ->
-                begin match
-                  betree_node_apply_upserts_fwd (BetreeListCons (k,
-                    BetreeMessageUpsert ufs) l) v key st2 with
-                | Fail e -> Fail e
-                | Return (st3, _) ->
-                  begin match
-                    betree_internal_lookup_in_children_back node key st1 st3
-                    with
-                  | Fail e -> Fail e
-                  | Return (st4, node0) ->
-                    begin match
-                      betree_node_apply_upserts_back (BetreeListCons (k,
-                        BetreeMessageUpsert ufs) l) v key st2 st4 with
-                    | Fail e -> Fail e
-                    | Return (st5, pending0) ->
-                      begin match
-                        betree_node_lookup_first_message_for_key_back key msgs
-                          pending0 with
-                      | Fail e -> Fail e
-                      | Return msgs0 ->
-                        begin match
-                          betree_store_internal_node_fwd
-                            node0.betree_internal_id msgs0 st5 with
-                        | Fail e -> Fail e
-                        | Return _ -> Return (st0, BetreeNodeInternal node0)
-                        end
-                      end
-                    end
-                  end
-                end
-              end
-            end
-        | BetreeListNil ->
-          begin match
+    let* (st1, msgs) = betree_load_internal_node_fwd node.betree_internal_id st
+      in
+    let* pending = betree_node_lookup_first_message_for_key_fwd key msgs in
+    begin match pending with
+    | BetreeListCons p l ->
+      let (k, msg) = p in
+      if k <> key
+      then begin
+        let* _ =
+          betree_node_lookup_first_message_for_key_back key msgs
+            (BetreeListCons (k, msg) l) in
+        let* (st2, node0) =
+          betree_internal_lookup_in_children_back node key st1 st0 in
+        Return (st2, BetreeNodeInternal node0) end
+      else
+        begin match msg with
+        | BetreeMessageInsert v ->
+          let* _ =
+            betree_node_lookup_first_message_for_key_back key msgs
+              (BetreeListCons (k, BetreeMessageInsert v) l) in
+          Return (st0, BetreeNodeInternal node)
+        | BetreeMessageDelete ->
+          let* _ =
             betree_node_lookup_first_message_for_key_back key msgs
-              BetreeListNil with
-          | Fail e -> Fail e
-          | Return _ ->
-            begin match
-              betree_internal_lookup_in_children_back node key st1 st0 with
-            | Fail e -> Fail e
-            | Return (st2, node0) -> Return (st2, BetreeNodeInternal node0)
-            end
-          end
+              (BetreeListCons (k, BetreeMessageDelete) l) in
+          Return (st0, BetreeNodeInternal node)
+        | BetreeMessageUpsert ufs ->
+          let* (st2, v) = betree_internal_lookup_in_children_fwd node key st1
+            in
+          let* (st3, _) =
+            betree_node_apply_upserts_fwd (BetreeListCons (k,
+              BetreeMessageUpsert ufs) l) v key st2 in
+          let* (st4, node0) =
+            betree_internal_lookup_in_children_back node key st1 st3 in
+          let* (st5, pending0) =
+            betree_node_apply_upserts_back (BetreeListCons (k,
+              BetreeMessageUpsert ufs) l) v key st2 st4 in
+          let* msgs0 =
+            betree_node_lookup_first_message_for_key_back key msgs pending0 in
+          let* _ =
+            betree_store_internal_node_fwd node0.betree_internal_id msgs0 st5
+            in
+          Return (st0, BetreeNodeInternal node0)
         end
-      end
+    | BetreeListNil ->
+      let* _ =
+        betree_node_lookup_first_message_for_key_back key msgs BetreeListNil in
+      let* (st2, node0) =
+        betree_internal_lookup_in_children_back node key st1 st0 in
+      Return (st2, BetreeNodeInternal node0)
     end
   | BetreeNodeLeaf node ->
-    begin match betree_load_leaf_node_fwd node.betree_leaf_id st with
-    | Fail e -> Fail e
-    | Return (_, bindings) ->
-      begin match betree_node_lookup_in_bindings_fwd key bindings with
-      | Fail e -> Fail e
-      | Return _ -> Return (st0, BetreeNodeLeaf node)
-      end
-    end
+    let* (_, bindings) = betree_load_leaf_node_fwd node.betree_leaf_id st in
+    let* _ = betree_node_lookup_in_bindings_fwd key bindings in
+    Return (st0, BetreeNodeLeaf node)
   end
 
 (** [betree_main::betree::Internal::{4}::lookup_in_children] *)
@@ -742,22 +503,16 @@ and betree_internal_lookup_in_children_back
   (decreases (betree_internal_lookup_in_children_decreases self key st))
   =
   if key < self.betree_internal_pivot
-  then
-    begin match betree_node_lookup_back self.betree_internal_left key st st0
-      with
-    | Fail e -> Fail e
-    | Return (st1, n) ->
-      Return (st1, Mkbetree_internal_t self.betree_internal_id
-        self.betree_internal_pivot n self.betree_internal_right)
-    end
-  else
-    begin match betree_node_lookup_back self.betree_internal_right key st st0
-      with
-    | Fail e -> Fail e
-    | Return (st1, n) ->
-      Return (st1, Mkbetree_internal_t self.betree_internal_id
-        self.betree_internal_pivot self.betree_internal_left n)
-    end
+  then begin
+    let* (st1, n) =
+      betree_node_lookup_back self.betree_internal_left key st st0 in
+    Return (st1, Mkbetree_internal_t self.betree_internal_id
+      self.betree_internal_pivot n self.betree_internal_right) end
+  else begin
+    let* (st1, n) =
+      betree_node_lookup_back self.betree_internal_right key st st0 in
+    Return (st1, Mkbetree_internal_t self.betree_internal_id
+      self.betree_internal_pivot self.betree_internal_left n) end
 
 (** [betree_main::betree::Node::{5}::lookup_mut_in_bindings] *)
 let rec betree_node_lookup_mut_in_bindings_fwd
@@ -786,11 +541,9 @@ let rec betree_node_lookup_mut_in_bindings_back
     let (i, i0) = hd in
     if i >= key
     then Return ret
-    else
-      begin match betree_node_lookup_mut_in_bindings_back key tl ret with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (BetreeListCons (i, i0) tl0)
-      end
+    else begin
+      let* tl0 = betree_node_lookup_mut_in_bindings_back key tl ret in
+      Return (BetreeListCons (i, i0) tl0) end
   | BetreeListNil -> Return ret
   end
 
@@ -800,82 +553,42 @@ let betree_node_apply_to_leaf_fwd_back
   (new_msg : betree_message_t) :
   result (betree_list_t (u64 & u64))
   =
-  begin match betree_node_lookup_mut_in_bindings_fwd key bindings with
-  | Fail e -> Fail e
-  | Return bindings0 ->
-    begin match betree_list_head_has_key_fwd u64 bindings0 key with
-    | Fail e -> Fail e
-    | Return b ->
-      if b
-      then
-        begin match betree_list_pop_front_fwd (u64 & u64) bindings0 with
-        | Fail e -> Fail e
-        | Return hd ->
-          begin match new_msg with
-          | BetreeMessageInsert v ->
-            begin match betree_list_pop_front_back (u64 & u64) bindings0 with
-            | Fail e -> Fail e
-            | Return bindings1 ->
-              begin match
-                betree_list_push_front_fwd_back (u64 & u64) bindings1 (key, v)
-                with
-              | Fail e -> Fail e
-              | Return bindings2 ->
-                betree_node_lookup_mut_in_bindings_back key bindings bindings2
-              end
-            end
-          | BetreeMessageDelete ->
-            begin match betree_list_pop_front_back (u64 & u64) bindings0 with
-            | Fail e -> Fail e
-            | Return bindings1 ->
-              betree_node_lookup_mut_in_bindings_back key bindings bindings1
-            end
-          | BetreeMessageUpsert s ->
-            let (_, i) = hd in
-            begin match betree_upsert_update_fwd (Some i) s with
-            | Fail e -> Fail e
-            | Return v ->
-              begin match betree_list_pop_front_back (u64 & u64) bindings0 with
-              | Fail e -> Fail e
-              | Return bindings1 ->
-                begin match
-                  betree_list_push_front_fwd_back (u64 & u64) bindings1 (key,
-                    v) with
-                | Fail e -> Fail e
-                | Return bindings2 ->
-                  betree_node_lookup_mut_in_bindings_back key bindings
-                    bindings2
-                end
-              end
-            end
-          end
-        end
-      else
-        begin match new_msg with
-        | BetreeMessageInsert v ->
-          begin match
-            betree_list_push_front_fwd_back (u64 & u64) bindings0 (key, v) with
-          | Fail e -> Fail e
-          | Return bindings1 ->
-            betree_node_lookup_mut_in_bindings_back key bindings bindings1
-          end
-        | BetreeMessageDelete ->
-          betree_node_lookup_mut_in_bindings_back key bindings bindings0
-        | BetreeMessageUpsert s ->
-          begin match betree_upsert_update_fwd None s with
-          | Fail e -> Fail e
-          | Return v ->
-            begin match
-              betree_list_push_front_fwd_back (u64 & u64) bindings0 (key, v)
-              with
-            | Fail e -> Fail e
-            | Return bindings1 ->
-              betree_node_lookup_mut_in_bindings_back key bindings bindings1
-            end
-          end
-        end
+  let* bindings0 = betree_node_lookup_mut_in_bindings_fwd key bindings in
+  let* b = betree_list_head_has_key_fwd u64 bindings0 key in
+  if b
+  then begin
+    let* hd = betree_list_pop_front_fwd (u64 & u64) bindings0 in
+    begin match new_msg with
+    | BetreeMessageInsert v ->
+      let* bindings1 = betree_list_pop_front_back (u64 & u64) bindings0 in
+      let* bindings2 =
+        betree_list_push_front_fwd_back (u64 & u64) bindings1 (key, v) in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings2
+    | BetreeMessageDelete ->
+      let* bindings1 = betree_list_pop_front_back (u64 & u64) bindings0 in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings1
+    | BetreeMessageUpsert s ->
+      let (_, i) = hd in
+      let* v = betree_upsert_update_fwd (Some i) s in
+      let* bindings1 = betree_list_pop_front_back (u64 & u64) bindings0 in
+      let* bindings2 =
+        betree_list_push_front_fwd_back (u64 & u64) bindings1 (key, v) in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings2
+    end end
+  else
+    begin match new_msg with
+    | BetreeMessageInsert v ->
+      let* bindings1 =
+        betree_list_push_front_fwd_back (u64 & u64) bindings0 (key, v) in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings1
+    | BetreeMessageDelete ->
+      betree_node_lookup_mut_in_bindings_back key bindings bindings0
+    | BetreeMessageUpsert s ->
+      let* v = betree_upsert_update_fwd None s in
+      let* bindings1 =
+        betree_list_push_front_fwd_back (u64 & u64) bindings0 (key, v) in
+      betree_node_lookup_mut_in_bindings_back key bindings bindings1
     end
-  end
 
 (** [betree_main::betree::Node::{5}::apply_messages_to_leaf] *)
 let rec betree_node_apply_messages_to_leaf_fwd_back
@@ -887,11 +600,8 @@ let rec betree_node_apply_messages_to_leaf_fwd_back
   begin match new_msgs with
   | BetreeListCons new_msg new_msgs_tl ->
     let (i, m) = new_msg in
-    begin match betree_node_apply_to_leaf_fwd_back bindings i m with
-    | Fail e -> Fail e
-    | Return bindings0 ->
-      betree_node_apply_messages_to_leaf_fwd_back bindings0 new_msgs_tl
-    end
+    let* bindings0 = betree_node_apply_to_leaf_fwd_back bindings i m in
+    betree_node_apply_messages_to_leaf_fwd_back bindings0 new_msgs_tl
   | BetreeListNil -> Return bindings
   end
 
@@ -905,13 +615,11 @@ let rec betree_node_filter_messages_for_key_fwd_back
   | BetreeListCons p l ->
     let (k, m) = p in
     if k = key
-    then
-      begin match
+    then begin
+      let* msgs0 =
         betree_list_pop_front_back (u64 & betree_message_t) (BetreeListCons (k,
-          m) l) with
-      | Fail e -> Fail e
-      | Return msgs0 -> betree_node_filter_messages_for_key_fwd_back key msgs0
-      end
+          m) l) in
+      betree_node_filter_messages_for_key_fwd_back key msgs0 end
     else Return (BetreeListCons (k, m) l)
   | BetreeListNil -> Return BetreeListNil
   end
@@ -942,12 +650,10 @@ let rec betree_node_lookup_first_message_after_key_back
   | BetreeListCons p next_msgs ->
     let (k, m) = p in
     if k = key
-    then
-      begin match
-        betree_node_lookup_first_message_after_key_back key next_msgs ret with
-      | Fail e -> Fail e
-      | Return next_msgs0 -> Return (BetreeListCons (k, m) next_msgs0)
-      end
+    then begin
+      let* next_msgs0 =
+        betree_node_lookup_first_message_after_key_back key next_msgs ret in
+      Return (BetreeListCons (k, m) next_msgs0) end
     else Return ret
   | BetreeListNil -> Return ret
   end
@@ -958,118 +664,59 @@ let betree_node_apply_to_internal_fwd_back
   (new_msg : betree_message_t) :
   result (betree_list_t (u64 & betree_message_t))
   =
-  begin match betree_node_lookup_first_message_for_key_fwd key msgs with
-  | Fail e -> Fail e
-  | Return msgs0 ->
-    begin match betree_list_head_has_key_fwd betree_message_t msgs0 key with
-    | Fail e -> Fail e
-    | Return b ->
-      if b
-      then
-        begin match new_msg with
-        | BetreeMessageInsert i ->
-          begin match betree_node_filter_messages_for_key_fwd_back key msgs0
-            with
-          | Fail e -> Fail e
-          | Return msgs1 ->
-            begin match
-              betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1
-                (key, BetreeMessageInsert i) with
-            | Fail e -> Fail e
-            | Return msgs2 ->
-              betree_node_lookup_first_message_for_key_back key msgs msgs2
-            end
-          end
-        | BetreeMessageDelete ->
-          begin match betree_node_filter_messages_for_key_fwd_back key msgs0
-            with
-          | Fail e -> Fail e
-          | Return msgs1 ->
-            begin match
-              betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1
-                (key, BetreeMessageDelete) with
-            | Fail e -> Fail e
-            | Return msgs2 ->
-              betree_node_lookup_first_message_for_key_back key msgs msgs2
-            end
-          end
-        | BetreeMessageUpsert s ->
-          begin match betree_list_hd_fwd (u64 & betree_message_t) msgs0 with
-          | Fail e -> Fail e
-          | Return p ->
-            let (_, m) = p in
-            begin match m with
-            | BetreeMessageInsert prev ->
-              begin match betree_upsert_update_fwd (Some prev) s with
-              | Fail e -> Fail e
-              | Return v ->
-                begin match
-                  betree_list_pop_front_back (u64 & betree_message_t) msgs0
-                  with
-                | Fail e -> Fail e
-                | Return msgs1 ->
-                  begin match
-                    betree_list_push_front_fwd_back (u64 & betree_message_t)
-                      msgs1 (key, BetreeMessageInsert v) with
-                  | Fail e -> Fail e
-                  | Return msgs2 ->
-                    betree_node_lookup_first_message_for_key_back key msgs
-                      msgs2
-                  end
-                end
-              end
-            | BetreeMessageDelete ->
-              begin match betree_upsert_update_fwd None s with
-              | Fail e -> Fail e
-              | Return v ->
-                begin match
-                  betree_list_pop_front_back (u64 & betree_message_t) msgs0
-                  with
-                | Fail e -> Fail e
-                | Return msgs1 ->
-                  begin match
-                    betree_list_push_front_fwd_back (u64 & betree_message_t)
-                      msgs1 (key, BetreeMessageInsert v) with
-                  | Fail e -> Fail e
-                  | Return msgs2 ->
-                    betree_node_lookup_first_message_for_key_back key msgs
-                      msgs2
-                  end
-                end
-              end
-            | BetreeMessageUpsert ufs ->
-              begin match
-                betree_node_lookup_first_message_after_key_fwd key msgs0 with
-              | Fail e -> Fail e
-              | Return msgs1 ->
-                begin match
-                  betree_list_push_front_fwd_back (u64 & betree_message_t)
-                    msgs1 (key, BetreeMessageUpsert s) with
-                | Fail e -> Fail e
-                | Return msgs2 ->
-                  begin match
-                    betree_node_lookup_first_message_after_key_back key msgs0
-                      msgs2 with
-                  | Fail e -> Fail e
-                  | Return msgs3 ->
-                    betree_node_lookup_first_message_for_key_back key msgs
-                      msgs3
-                  end
-                end
-              end
-            end
-          end
-        end
-      else
-        begin match
-          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs0 (key,
-            new_msg) with
-        | Fail e -> Fail e
-        | Return msgs1 ->
-          betree_node_lookup_first_message_for_key_back key msgs msgs1
-        end
+  let* msgs0 = betree_node_lookup_first_message_for_key_fwd key msgs in
+  let* b = betree_list_head_has_key_fwd betree_message_t msgs0 key in
+  if b
+  then
+    begin match new_msg with
+    | BetreeMessageInsert i ->
+      let* msgs1 = betree_node_filter_messages_for_key_fwd_back key msgs0 in
+      let* msgs2 =
+        betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+          BetreeMessageInsert i) in
+      betree_node_lookup_first_message_for_key_back key msgs msgs2
+    | BetreeMessageDelete ->
+      let* msgs1 = betree_node_filter_messages_for_key_fwd_back key msgs0 in
+      let* msgs2 =
+        betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+          BetreeMessageDelete) in
+      betree_node_lookup_first_message_for_key_back key msgs msgs2
+    | BetreeMessageUpsert s ->
+      let* p = betree_list_hd_fwd (u64 & betree_message_t) msgs0 in
+      let (_, m) = p in
+      begin match m with
+      | BetreeMessageInsert prev ->
+        let* v = betree_upsert_update_fwd (Some prev) s in
+        let* msgs1 = betree_list_pop_front_back (u64 & betree_message_t) msgs0
+          in
+        let* msgs2 =
+          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+            BetreeMessageInsert v) in
+        betree_node_lookup_first_message_for_key_back key msgs msgs2
+      | BetreeMessageDelete ->
+        let* v = betree_upsert_update_fwd None s in
+        let* msgs1 = betree_list_pop_front_back (u64 & betree_message_t) msgs0
+          in
+        let* msgs2 =
+          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+            BetreeMessageInsert v) in
+        betree_node_lookup_first_message_for_key_back key msgs msgs2
+      | BetreeMessageUpsert ufs ->
+        let* msgs1 = betree_node_lookup_first_message_after_key_fwd key msgs0
+          in
+        let* msgs2 =
+          betree_list_push_front_fwd_back (u64 & betree_message_t) msgs1 (key,
+            BetreeMessageUpsert s) in
+        let* msgs3 =
+          betree_node_lookup_first_message_after_key_back key msgs0 msgs2 in
+        betree_node_lookup_first_message_for_key_back key msgs msgs3
+      end
     end
-  end
+  else begin
+    let* msgs1 =
+      betree_list_push_front_fwd_back (u64 & betree_message_t) msgs0 (key,
+        new_msg) in
+    betree_node_lookup_first_message_for_key_back key msgs msgs1 end
 
 (** [betree_main::betree::Node::{5}::apply_messages_to_internal] *)
 let rec betree_node_apply_messages_to_internal_fwd_back
@@ -1081,11 +728,8 @@ let rec betree_node_apply_messages_to_internal_fwd_back
   begin match new_msgs with
   | BetreeListCons new_msg new_msgs_tl ->
     let (i, m) = new_msg in
-    begin match betree_node_apply_to_internal_fwd_back msgs i m with
-    | Fail e -> Fail e
-    | Return msgs0 ->
-      betree_node_apply_messages_to_internal_fwd_back msgs0 new_msgs_tl
-    end
+    let* msgs0 = betree_node_apply_to_internal_fwd_back msgs i m in
+    betree_node_apply_messages_to_internal_fwd_back msgs0 new_msgs_tl
   | BetreeListNil -> Return msgs
   end
 
@@ -1100,85 +744,41 @@ let rec betree_node_apply_messages_fwd
   =
   begin match self with
   | BetreeNodeInternal node ->
-    begin match betree_load_internal_node_fwd node.betree_internal_id st with
-    | Fail e -> Fail e
-    | Return (st0, content) ->
-      begin match betree_node_apply_messages_to_internal_fwd_back content msgs
-        with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & betree_message_t) content0 with
-        | Fail e -> Fail e
-        | Return num_msgs ->
-          if num_msgs >= params.betree_params_min_flush_size
-          then
-            begin match
-              betree_internal_flush_fwd node params node_id_cnt content0 st0
-              with
-            | Fail e -> Fail e
-            | Return (st1, content1) ->
-              begin match
-                betree_internal_flush_back'a node params node_id_cnt content0
-                  st0 st1 with
-              | Fail e -> Fail e
-              | Return (st2, (node0, _)) ->
-                begin match
-                  betree_store_internal_node_fwd node0.betree_internal_id
-                    content1 st2 with
-                | Fail e -> Fail e
-                | Return (st3, _) -> Return (st3, ())
-                end
-              end
-            end
-          else
-            begin match
-              betree_store_internal_node_fwd node.betree_internal_id content0
-                st0 with
-            | Fail e -> Fail e
-            | Return (st1, _) -> Return (st1, ())
-            end
-        end
-      end
-    end
+    let* (st0, content) =
+      betree_load_internal_node_fwd node.betree_internal_id st in
+    let* content0 =
+      betree_node_apply_messages_to_internal_fwd_back content msgs in
+    let* num_msgs = betree_list_len_fwd (u64 & betree_message_t) content0 in
+    if num_msgs >= params.betree_params_min_flush_size
+    then begin
+      let* (st1, content1) =
+        betree_internal_flush_fwd node params node_id_cnt content0 st0 in
+      let* (st2, (node0, _)) =
+        betree_internal_flush_back'a node params node_id_cnt content0 st0 st1
+        in
+      let* (st3, _) =
+        betree_store_internal_node_fwd node0.betree_internal_id content1 st2 in
+      Return (st3, ()) end
+    else begin
+      let* (st1, _) =
+        betree_store_internal_node_fwd node.betree_internal_id content0 st0 in
+      Return (st1, ()) end
   | BetreeNodeLeaf node ->
-    begin match betree_load_leaf_node_fwd node.betree_leaf_id st with
-    | Fail e -> Fail e
-    | Return (st0, content) ->
-      begin match betree_node_apply_messages_to_leaf_fwd_back content msgs with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & u64) content0 with
-        | Fail e -> Fail e
-        | Return len ->
-          begin match u64_mul 2 params.betree_params_split_size with
-          | Fail e -> Fail e
-          | Return i ->
-            if len >= i
-            then
-              begin match
-                betree_leaf_split_fwd node content0 params node_id_cnt st0 with
-              | Fail e -> Fail e
-              | Return (st1, _) ->
-                begin match
-                  betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil
-                    st1 with
-                | Fail e -> Fail e
-                | Return (st2, _) ->
-                  betree_leaf_split_back0 node content0 params node_id_cnt st0
-                    st2
-                end
-              end
-            else
-              begin match
-                betree_store_leaf_node_fwd node.betree_leaf_id content0 st0
-                with
-              | Fail e -> Fail e
-              | Return (st1, _) -> Return (st1, ())
-              end
-          end
-        end
-      end
-    end
+    let* (st0, content) = betree_load_leaf_node_fwd node.betree_leaf_id st in
+    let* content0 = betree_node_apply_messages_to_leaf_fwd_back content msgs in
+    let* len = betree_list_len_fwd (u64 & u64) content0 in
+    let* i = u64_mul 2 params.betree_params_split_size in
+    if len >= i
+    then begin
+      let* (st1, _) =
+        betree_leaf_split_fwd node content0 params node_id_cnt st0 in
+      let* (st2, _) =
+        betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil st1 in
+      betree_leaf_split_back0 node content0 params node_id_cnt st0 st2 end
+    else begin
+      let* (st1, _) =
+        betree_store_leaf_node_fwd node.betree_leaf_id content0 st0 in
+      Return (st1, ()) end
   end
 
 (** [betree_main::betree::Node::{5}::apply_messages] *)
@@ -1192,99 +792,45 @@ and betree_node_apply_messages_back'a
   =
   begin match self with
   | BetreeNodeInternal node ->
-    begin match betree_load_internal_node_fwd node.betree_internal_id st with
-    | Fail e -> Fail e
-    | Return (st1, content) ->
-      begin match betree_node_apply_messages_to_internal_fwd_back content msgs
-        with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & betree_message_t) content0 with
-        | Fail e -> Fail e
-        | Return num_msgs ->
-          if num_msgs >= params.betree_params_min_flush_size
-          then
-            begin match
-              betree_internal_flush_fwd node params node_id_cnt content0 st1
-              with
-            | Fail e -> Fail e
-            | Return (st2, content1) ->
-              begin match
-                betree_internal_flush_back'a node params node_id_cnt content0
-                  st1 st2 with
-              | Fail e -> Fail e
-              | Return (st3, (node0, node_id_cnt0)) ->
-                begin match
-                  betree_store_internal_node_fwd node0.betree_internal_id
-                    content1 st3 with
-                | Fail e -> Fail e
-                | Return _ ->
-                  Return (st0, (BetreeNodeInternal node0, node_id_cnt0))
-                end
-              end
-            end
-          else
-            begin match
-              betree_store_internal_node_fwd node.betree_internal_id content0
-                st1 with
-            | Fail e -> Fail e
-            | Return _ -> Return (st0, (BetreeNodeInternal node, node_id_cnt))
-            end
-        end
-      end
-    end
+    let* (st1, content) =
+      betree_load_internal_node_fwd node.betree_internal_id st in
+    let* content0 =
+      betree_node_apply_messages_to_internal_fwd_back content msgs in
+    let* num_msgs = betree_list_len_fwd (u64 & betree_message_t) content0 in
+    if num_msgs >= params.betree_params_min_flush_size
+    then begin
+      let* (st2, content1) =
+        betree_internal_flush_fwd node params node_id_cnt content0 st1 in
+      let* (st3, (node0, node_id_cnt0)) =
+        betree_internal_flush_back'a node params node_id_cnt content0 st1 st2
+        in
+      let* _ =
+        betree_store_internal_node_fwd node0.betree_internal_id content1 st3 in
+      Return (st0, (BetreeNodeInternal node0, node_id_cnt0)) end
+    else begin
+      let* _ =
+        betree_store_internal_node_fwd node.betree_internal_id content0 st1 in
+      Return (st0, (BetreeNodeInternal node, node_id_cnt)) end
   | BetreeNodeLeaf node ->
-    begin match betree_load_leaf_node_fwd node.betree_leaf_id st with
-    | Fail e -> Fail e
-    | Return (st1, content) ->
-      begin match betree_node_apply_messages_to_leaf_fwd_back content msgs with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & u64) content0 with
-        | Fail e -> Fail e
-        | Return len ->
-          begin match u64_mul 2 params.betree_params_split_size with
-          | Fail e -> Fail e
-          | Return i ->
-            if len >= i
-            then
-              begin match
-                betree_leaf_split_fwd node content0 params node_id_cnt st1 with
-              | Fail e -> Fail e
-              | Return (st2, new_node) ->
-                begin match
-                  betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil
-                    st2 with
-                | Fail e -> Fail e
-                | Return (st3, _) ->
-                  begin match
-                    betree_leaf_split_back0 node content0 params node_id_cnt
-                      st1 st3 with
-                  | Fail e -> Fail e
-                  | Return _ ->
-                    begin match
-                      betree_leaf_split_back2 node content0 params node_id_cnt
-                        st1 st0 with
-                    | Fail e -> Fail e
-                    | Return (st4, node_id_cnt0) ->
-                      Return (st4, (BetreeNodeInternal new_node, node_id_cnt0))
-                    end
-                  end
-                end
-              end
-            else
-              begin match
-                betree_store_leaf_node_fwd node.betree_leaf_id content0 st1
-                with
-              | Fail e -> Fail e
-              | Return _ ->
-                Return (st0, (BetreeNodeLeaf (Mkbetree_leaf_t
-                  node.betree_leaf_id len), node_id_cnt))
-              end
-          end
-        end
-      end
-    end
+    let* (st1, content) = betree_load_leaf_node_fwd node.betree_leaf_id st in
+    let* content0 = betree_node_apply_messages_to_leaf_fwd_back content msgs in
+    let* len = betree_list_len_fwd (u64 & u64) content0 in
+    let* i = u64_mul 2 params.betree_params_split_size in
+    if len >= i
+    then begin
+      let* (st2, new_node) =
+        betree_leaf_split_fwd node content0 params node_id_cnt st1 in
+      let* (st3, _) =
+        betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil st2 in
+      let* _ = betree_leaf_split_back0 node content0 params node_id_cnt st1 st3
+        in
+      let* (st4, node_id_cnt0) =
+        betree_leaf_split_back2 node content0 params node_id_cnt st1 st0 in
+      Return (st4, (BetreeNodeInternal new_node, node_id_cnt0)) end
+    else begin
+      let* _ = betree_store_leaf_node_fwd node.betree_leaf_id content0 st1 in
+      Return (st0, (BetreeNodeLeaf (Mkbetree_leaf_t node.betree_leaf_id len),
+        node_id_cnt)) end
   end
 
 (** [betree_main::betree::Node::{5}::apply_messages] *)
@@ -1298,93 +844,42 @@ and betree_node_apply_messages_back1
   =
   begin match self with
   | BetreeNodeInternal node ->
-    begin match betree_load_internal_node_fwd node.betree_internal_id st with
-    | Fail e -> Fail e
-    | Return (st1, content) ->
-      begin match betree_node_apply_messages_to_internal_fwd_back content msgs
-        with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & betree_message_t) content0 with
-        | Fail e -> Fail e
-        | Return num_msgs ->
-          if num_msgs >= params.betree_params_min_flush_size
-          then
-            begin match
-              betree_internal_flush_fwd node params node_id_cnt content0 st1
-              with
-            | Fail e -> Fail e
-            | Return (st2, content1) ->
-              begin match
-                betree_internal_flush_back'a node params node_id_cnt content0
-                  st1 st2 with
-              | Fail e -> Fail e
-              | Return (st3, (node0, _)) ->
-                begin match
-                  betree_store_internal_node_fwd node0.betree_internal_id
-                    content1 st3 with
-                | Fail e -> Fail e
-                | Return _ ->
-                  betree_internal_flush_back1 node params node_id_cnt content0
-                    st1 st0
-                end
-              end
-            end
-          else
-            begin match
-              betree_store_internal_node_fwd node.betree_internal_id content0
-                st1 with
-            | Fail e -> Fail e
-            | Return _ -> Return (st0, ())
-            end
-        end
-      end
-    end
+    let* (st1, content) =
+      betree_load_internal_node_fwd node.betree_internal_id st in
+    let* content0 =
+      betree_node_apply_messages_to_internal_fwd_back content msgs in
+    let* num_msgs = betree_list_len_fwd (u64 & betree_message_t) content0 in
+    if num_msgs >= params.betree_params_min_flush_size
+    then begin
+      let* (st2, content1) =
+        betree_internal_flush_fwd node params node_id_cnt content0 st1 in
+      let* (st3, (node0, _)) =
+        betree_internal_flush_back'a node params node_id_cnt content0 st1 st2
+        in
+      let* _ =
+        betree_store_internal_node_fwd node0.betree_internal_id content1 st3 in
+      betree_internal_flush_back1 node params node_id_cnt content0 st1 st0 end
+    else begin
+      let* _ =
+        betree_store_internal_node_fwd node.betree_internal_id content0 st1 in
+      Return (st0, ()) end
   | BetreeNodeLeaf node ->
-    begin match betree_load_leaf_node_fwd node.betree_leaf_id st with
-    | Fail e -> Fail e
-    | Return (st1, content) ->
-      begin match betree_node_apply_messages_to_leaf_fwd_back content msgs with
-      | Fail e -> Fail e
-      | Return content0 ->
-        begin match betree_list_len_fwd (u64 & u64) content0 with
-        | Fail e -> Fail e
-        | Return len ->
-          begin match u64_mul 2 params.betree_params_split_size with
-          | Fail e -> Fail e
-          | Return i ->
-            if len >= i
-            then
-              begin match
-                betree_leaf_split_fwd node content0 params node_id_cnt st1 with
-              | Fail e -> Fail e
-              | Return (st2, _) ->
-                begin match
-                  betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil
-                    st2 with
-                | Fail e -> Fail e
-                | Return (st3, _) ->
-                  begin match
-                    betree_leaf_split_back0 node content0 params node_id_cnt
-                      st1 st3 with
-                  | Fail e -> Fail e
-                  | Return _ ->
-                    betree_leaf_split_back1 node content0 params node_id_cnt
-                      st1 st0
-                  end
-                end
-              end
-            else
-              begin match
-                betree_store_leaf_node_fwd node.betree_leaf_id content0 st1
-                with
-              | Fail e -> Fail e
-              | Return _ -> Return (st0, ())
-              end
-          end
-        end
-      end
-    end
+    let* (st1, content) = betree_load_leaf_node_fwd node.betree_leaf_id st in
+    let* content0 = betree_node_apply_messages_to_leaf_fwd_back content msgs in
+    let* len = betree_list_len_fwd (u64 & u64) content0 in
+    let* i = u64_mul 2 params.betree_params_split_size in
+    if len >= i
+    then begin
+      let* (st2, _) =
+        betree_leaf_split_fwd node content0 params node_id_cnt st1 in
+      let* (st3, _) =
+        betree_store_leaf_node_fwd node.betree_leaf_id BetreeListNil st2 in
+      let* _ = betree_leaf_split_back0 node content0 params node_id_cnt st1 st3
+        in
+      betree_leaf_split_back1 node content0 params node_id_cnt st1 st0 end
+    else begin
+      let* _ = betree_store_leaf_node_fwd node.betree_leaf_id content0 st1 in
+      Return (st0, ()) end
   end
 
 (** [betree_main::betree::Internal::{4}::flush] *)
@@ -1396,84 +891,47 @@ and betree_internal_flush_fwd
   (decreases (
     betree_internal_flush_decreases self params node_id_cnt content st))
   =
-  begin match
+  let* p =
     betree_list_partition_at_pivot_fwd betree_message_t content
-      self.betree_internal_pivot with
-  | Fail e -> Fail e
-  | Return p ->
-    let (msgs_left, msgs_right) = p in
-    begin match betree_list_len_fwd (u64 & betree_message_t) msgs_left with
-    | Fail e -> Fail e
-    | Return len_left ->
-      if len_left >= params.betree_params_min_flush_size
-      then
-        begin match
-          betree_node_apply_messages_fwd self.betree_internal_left params
-            node_id_cnt msgs_left st with
-        | Fail e -> Fail e
-        | Return (st0, _) ->
-          begin match
-            betree_node_apply_messages_back'a self.betree_internal_left params
-              node_id_cnt msgs_left st st0 with
-          | Fail e -> Fail e
-          | Return (st1, (_, node_id_cnt0)) ->
-            begin match
-              betree_node_apply_messages_back1 self.betree_internal_left params
-                node_id_cnt msgs_left st st1 with
-            | Fail e -> Fail e
-            | Return (st2, ()) ->
-              begin match
-                betree_list_len_fwd (u64 & betree_message_t) msgs_right with
-              | Fail e -> Fail e
-              | Return len_right ->
-                if len_right >= params.betree_params_min_flush_size
-                then
-                  begin match
-                    betree_node_apply_messages_fwd self.betree_internal_right
-                      params node_id_cnt0 msgs_right st2 with
-                  | Fail e -> Fail e
-                  | Return (st3, _) ->
-                    begin match
-                      betree_node_apply_messages_back'a
-                        self.betree_internal_right params node_id_cnt0
-                        msgs_right st2 st3 with
-                    | Fail e -> Fail e
-                    | Return (st4, (_, _)) ->
-                      begin match
-                        betree_node_apply_messages_back1
-                          self.betree_internal_right params node_id_cnt0
-                          msgs_right st2 st4 with
-                      | Fail e -> Fail e
-                      | Return (st5, ()) -> Return (st5, BetreeListNil)
-                      end
-                    end
-                  end
-                else Return (st2, msgs_right)
-              end
-            end
-          end
-        end
-      else
-        begin match
-          betree_node_apply_messages_fwd self.betree_internal_right params
-            node_id_cnt msgs_right st with
-        | Fail e -> Fail e
-        | Return (st0, _) ->
-          begin match
-            betree_node_apply_messages_back'a self.betree_internal_right params
-              node_id_cnt msgs_right st st0 with
-          | Fail e -> Fail e
-          | Return (st1, (_, _)) ->
-            begin match
-              betree_node_apply_messages_back1 self.betree_internal_right
-                params node_id_cnt msgs_right st st1 with
-            | Fail e -> Fail e
-            | Return (st2, ()) -> Return (st2, msgs_left)
-            end
-          end
-        end
-    end
-  end
+      self.betree_internal_pivot in
+  let (msgs_left, msgs_right) = p in
+  let* len_left = betree_list_len_fwd (u64 & betree_message_t) msgs_left in
+  if len_left >= params.betree_params_min_flush_size
+  then begin
+    let* (st0, _) =
+      betree_node_apply_messages_fwd self.betree_internal_left params
+        node_id_cnt msgs_left st in
+    let* (st1, (_, node_id_cnt0)) =
+      betree_node_apply_messages_back'a self.betree_internal_left params
+        node_id_cnt msgs_left st st0 in
+    let* (st2, ()) =
+      betree_node_apply_messages_back1 self.betree_internal_left params
+        node_id_cnt msgs_left st st1 in
+    let* len_right = betree_list_len_fwd (u64 & betree_message_t) msgs_right in
+    if len_right >= params.betree_params_min_flush_size
+    then begin
+      let* (st3, _) =
+        betree_node_apply_messages_fwd self.betree_internal_right params
+          node_id_cnt0 msgs_right st2 in
+      let* (st4, (_, _)) =
+        betree_node_apply_messages_back'a self.betree_internal_right params
+          node_id_cnt0 msgs_right st2 st3 in
+      let* (st5, ()) =
+        betree_node_apply_messages_back1 self.betree_internal_right params
+          node_id_cnt0 msgs_right st2 st4 in
+      Return (st5, BetreeListNil) end
+    else Return (st2, msgs_right) end
+  else begin
+    let* (st0, _) =
+      betree_node_apply_messages_fwd self.betree_internal_right params
+        node_id_cnt msgs_right st in
+    let* (st1, (_, _)) =
+      betree_node_apply_messages_back'a self.betree_internal_right params
+        node_id_cnt msgs_right st st0 in
+    let* (st2, ()) =
+      betree_node_apply_messages_back1 self.betree_internal_right params
+        node_id_cnt msgs_right st st1 in
+    Return (st2, msgs_left) end
 
 (** [betree_main::betree::Internal::{4}::flush] *)
 and betree_internal_flush_back'a
@@ -1485,93 +943,53 @@ and betree_internal_flush_back'a
   (decreases (
     betree_internal_flush_decreases self params node_id_cnt content st))
   =
-  begin match
+  let* p =
     betree_list_partition_at_pivot_fwd betree_message_t content
-      self.betree_internal_pivot with
-  | Fail e -> Fail e
-  | Return p ->
-    let (msgs_left, msgs_right) = p in
-    begin match betree_list_len_fwd (u64 & betree_message_t) msgs_left with
-    | Fail e -> Fail e
-    | Return len_left ->
-      if len_left >= params.betree_params_min_flush_size
-      then
-        begin match
-          betree_node_apply_messages_fwd self.betree_internal_left params
-            node_id_cnt msgs_left st with
-        | Fail e -> Fail e
-        | Return (st1, _) ->
-          begin match
-            betree_node_apply_messages_back'a self.betree_internal_left params
-              node_id_cnt msgs_left st st1 with
-          | Fail e -> Fail e
-          | Return (st2, (n, node_id_cnt0)) ->
-            begin match
-              betree_node_apply_messages_back1 self.betree_internal_left params
-                node_id_cnt msgs_left st st2 with
-            | Fail e -> Fail e
-            | Return (st3, ()) ->
-              begin match
-                betree_list_len_fwd (u64 & betree_message_t) msgs_right with
-              | Fail e -> Fail e
-              | Return len_right ->
-                if len_right >= params.betree_params_min_flush_size
-                then
-                  begin match
-                    betree_node_apply_messages_fwd self.betree_internal_right
-                      params node_id_cnt0 msgs_right st3 with
-                  | Fail e -> Fail e
-                  | Return (st4, _) ->
-                    begin match
-                      betree_node_apply_messages_back'a
-                        self.betree_internal_right params node_id_cnt0
-                        msgs_right st3 st4 with
-                    | Fail e -> Fail e
-                    | Return (st5, (n0, node_id_cnt1)) ->
-                      begin match
-                        betree_node_apply_messages_back1
-                          self.betree_internal_right params node_id_cnt0
-                          msgs_right st3 st5 with
-                      | Fail e -> Fail e
-                      | Return _ ->
-                        Return (st0, (Mkbetree_internal_t
-                          self.betree_internal_id self.betree_internal_pivot n
-                          n0, node_id_cnt1))
-                      end
-                    end
-                  end
-                else
-                  Return (st0, (Mkbetree_internal_t self.betree_internal_id
-                    self.betree_internal_pivot n self.betree_internal_right,
-                    node_id_cnt0))
-              end
-            end
-          end
-        end
-      else
-        begin match
-          betree_node_apply_messages_fwd self.betree_internal_right params
-            node_id_cnt msgs_right st with
-        | Fail e -> Fail e
-        | Return (st1, _) ->
-          begin match
-            betree_node_apply_messages_back'a self.betree_internal_right params
-              node_id_cnt msgs_right st st1 with
-          | Fail e -> Fail e
-          | Return (st2, (n, node_id_cnt0)) ->
-            begin match
-              betree_node_apply_messages_back1 self.betree_internal_right
-                params node_id_cnt msgs_right st st2 with
-            | Fail e -> Fail e
-            | Return _ ->
-              Return (st0, (Mkbetree_internal_t self.betree_internal_id
-                self.betree_internal_pivot self.betree_internal_left n,
-                node_id_cnt0))
-            end
-          end
-        end
+      self.betree_internal_pivot in
+  let (msgs_left, msgs_right) = p in
+  let* len_left = betree_list_len_fwd (u64 & betree_message_t) msgs_left in
+  if len_left >= params.betree_params_min_flush_size
+  then begin
+    let* (st1, _) =
+      betree_node_apply_messages_fwd self.betree_internal_left params
+        node_id_cnt msgs_left st in
+    let* (st2, (n, node_id_cnt0)) =
+      betree_node_apply_messages_back'a self.betree_internal_left params
+        node_id_cnt msgs_left st st1 in
+    let* (st3, ()) =
+      betree_node_apply_messages_back1 self.betree_internal_left params
+        node_id_cnt msgs_left st st2 in
+    let* len_right = betree_list_len_fwd (u64 & betree_message_t) msgs_right in
+    if len_right >= params.betree_params_min_flush_size
+    then begin
+      let* (st4, _) =
+        betree_node_apply_messages_fwd self.betree_internal_right params
+          node_id_cnt0 msgs_right st3 in
+      let* (st5, (n0, node_id_cnt1)) =
+        betree_node_apply_messages_back'a self.betree_internal_right params
+          node_id_cnt0 msgs_right st3 st4 in
+      let* _ =
+        betree_node_apply_messages_back1 self.betree_internal_right params
+          node_id_cnt0 msgs_right st3 st5 in
+      Return (st0, (Mkbetree_internal_t self.betree_internal_id
+        self.betree_internal_pivot n n0, node_id_cnt1)) end
+    else
+      Return (st0, (Mkbetree_internal_t self.betree_internal_id
+        self.betree_internal_pivot n self.betree_internal_right, node_id_cnt0))
+    end
+  else begin
+    let* (st1, _) =
+      betree_node_apply_messages_fwd self.betree_internal_right params
+        node_id_cnt msgs_right st in
+    let* (st2, (n, node_id_cnt0)) =
+      betree_node_apply_messages_back'a self.betree_internal_right params
+        node_id_cnt msgs_right st st1 in
+    let* _ =
+      betree_node_apply_messages_back1 self.betree_internal_right params
+        node_id_cnt msgs_right st st2 in
+    Return (st0, (Mkbetree_internal_t self.betree_internal_id
+      self.betree_internal_pivot self.betree_internal_left n, node_id_cnt0))
     end
-  end
 
 (** [betree_main::betree::Internal::{4}::flush] *)
 and betree_internal_flush_back1
@@ -1583,84 +1001,47 @@ and betree_internal_flush_back1
   (decreases (
     betree_internal_flush_decreases self params node_id_cnt content st))
   =
-  begin match
+  let* p =
     betree_list_partition_at_pivot_fwd betree_message_t content
-      self.betree_internal_pivot with
-  | Fail e -> Fail e
-  | Return p ->
-    let (msgs_left, msgs_right) = p in
-    begin match betree_list_len_fwd (u64 & betree_message_t) msgs_left with
-    | Fail e -> Fail e
-    | Return len_left ->
-      if len_left >= params.betree_params_min_flush_size
-      then
-        begin match
-          betree_node_apply_messages_fwd self.betree_internal_left params
-            node_id_cnt msgs_left st with
-        | Fail e -> Fail e
-        | Return (st1, _) ->
-          begin match
-            betree_node_apply_messages_back'a self.betree_internal_left params
-              node_id_cnt msgs_left st st1 with
-          | Fail e -> Fail e
-          | Return (st2, (_, node_id_cnt0)) ->
-            begin match
-              betree_node_apply_messages_back1 self.betree_internal_left params
-                node_id_cnt msgs_left st st2 with
-            | Fail e -> Fail e
-            | Return (st3, ()) ->
-              begin match
-                betree_list_len_fwd (u64 & betree_message_t) msgs_right with
-              | Fail e -> Fail e
-              | Return len_right ->
-                if len_right >= params.betree_params_min_flush_size
-                then
-                  begin match
-                    betree_node_apply_messages_fwd self.betree_internal_right
-                      params node_id_cnt0 msgs_right st3 with
-                  | Fail e -> Fail e
-                  | Return (st4, _) ->
-                    begin match
-                      betree_node_apply_messages_back'a
-                        self.betree_internal_right params node_id_cnt0
-                        msgs_right st3 st4 with
-                    | Fail e -> Fail e
-                    | Return (st5, (_, _)) ->
-                      begin match
-                        betree_node_apply_messages_back1
-                          self.betree_internal_right params node_id_cnt0
-                          msgs_right st3 st5 with
-                      | Fail e -> Fail e
-                      | Return _ -> Return (st0, ())
-                      end
-                    end
-                  end
-                else Return (st0, ())
-              end
-            end
-          end
-        end
-      else
-        begin match
-          betree_node_apply_messages_fwd self.betree_internal_right params
-            node_id_cnt msgs_right st with
-        | Fail e -> Fail e
-        | Return (st1, _) ->
-          begin match
-            betree_node_apply_messages_back'a self.betree_internal_right params
-              node_id_cnt msgs_right st st1 with
-          | Fail e -> Fail e
-          | Return (st2, (_, _)) ->
-            begin match
-              betree_node_apply_messages_back1 self.betree_internal_right
-                params node_id_cnt msgs_right st st2 with
-            | Fail e -> Fail e
-            | Return _ -> Return (st0, ())
-            end
-          end
-        end
-    end
-  end
+      self.betree_internal_pivot in
+  let (msgs_left, msgs_right) = p in
+  let* len_left = betree_list_len_fwd (u64 & betree_message_t) msgs_left in
+  if len_left >= params.betree_params_min_flush_size
+  then begin
+    let* (st1, _) =
+      betree_node_apply_messages_fwd self.betree_internal_left params
+        node_id_cnt msgs_left st in
+    let* (st2, (_, node_id_cnt0)) =
+      betree_node_apply_messages_back'a self.betree_internal_left params
+        node_id_cnt msgs_left st st1 in
+    let* (st3, ()) =
+      betree_node_apply_messages_back1 self.betree_internal_left params
+        node_id_cnt msgs_left st st2 in
+    let* len_right = betree_list_len_fwd (u64 & betree_message_t) msgs_right in
+    if len_right >= params.betree_params_min_flush_size
+    then begin
+      let* (st4, _) =
+        betree_node_apply_messages_fwd self.betree_internal_right params
+          node_id_cnt0 msgs_right st3 in
+      let* (st5, (_, _)) =
+        betree_node_apply_messages_back'a self.betree_internal_right params
+          node_id_cnt0 msgs_right st3 st4 in
+      let* _ =
+        betree_node_apply_messages_back1 self.betree_internal_right params
+          node_id_cnt0 msgs_right st3 st5 in
+      Return (st0, ()) end
+    else Return (st0, ()) end
+  else begin
+    let* (st1, _) =
+      betree_node_apply_messages_fwd self.betree_internal_right params
+        node_id_cnt msgs_right st in
+    let* (st2, (_, _)) =
+      betree_node_apply_messages_back'a self.betree_internal_right params
+        node_id_cnt msgs_right st st1 in
+    let* _ =
+      betree_node_apply_messages_back1 self.betree_internal_right params
+        node_id_cnt msgs_right st st2 in
+    Return (st0, ()) end
 
 (** [betree_main::betree::Node::{5}::apply] *)
 let betree_node_apply_fwd
@@ -1670,20 +1051,14 @@ let betree_node_apply_fwd
   result (state & unit)
   =
   let l = BetreeListNil in
-  begin match
+  let* (st0, _) =
     betree_node_apply_messages_fwd self params node_id_cnt (BetreeListCons
-      (key, new_msg) l) st with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match
-      betree_node_apply_messages_back'a self params node_id_cnt (BetreeListCons
-        (key, new_msg) l) st st0 with
-    | Fail e -> Fail e
-    | Return (st1, (_, _)) ->
-      betree_node_apply_messages_back1 self params node_id_cnt (BetreeListCons
-        (key, new_msg) l) st st1
-    end
-  end
+      (key, new_msg) l) st in
+  let* (st1, (_, _)) =
+    betree_node_apply_messages_back'a self params node_id_cnt (BetreeListCons
+      (key, new_msg) l) st st0 in
+  betree_node_apply_messages_back1 self params node_id_cnt (BetreeListCons
+    (key, new_msg) l) st st1
 
 (** [betree_main::betree::Node::{5}::apply] *)
 let betree_node_apply_back'a
@@ -1693,24 +1068,16 @@ let betree_node_apply_back'a
   result (state & (betree_node_t & betree_node_id_counter_t))
   =
   let l = BetreeListNil in
-  begin match
+  let* (st1, _) =
     betree_node_apply_messages_fwd self params node_id_cnt (BetreeListCons
-      (key, new_msg) l) st with
-  | Fail e -> Fail e
-  | Return (st1, _) ->
-    begin match
-      betree_node_apply_messages_back'a self params node_id_cnt (BetreeListCons
-        (key, new_msg) l) st st1 with
-    | Fail e -> Fail e
-    | Return (st2, (self0, node_id_cnt0)) ->
-      begin match
-        betree_node_apply_messages_back1 self params node_id_cnt
-          (BetreeListCons (key, new_msg) l) st st2 with
-      | Fail e -> Fail e
-      | Return _ -> Return (st0, (self0, node_id_cnt0))
-      end
-    end
-  end
+      (key, new_msg) l) st in
+  let* (st2, (self0, node_id_cnt0)) =
+    betree_node_apply_messages_back'a self params node_id_cnt (BetreeListCons
+      (key, new_msg) l) st st1 in
+  let* _ =
+    betree_node_apply_messages_back1 self params node_id_cnt (BetreeListCons
+      (key, new_msg) l) st st2 in
+  Return (st0, (self0, node_id_cnt0))
 
 (** [betree_main::betree::Node::{5}::apply] *)
 let betree_node_apply_back1
@@ -1720,70 +1087,43 @@ let betree_node_apply_back1
   result (state & unit)
   =
   let l = BetreeListNil in
-  begin match
+  let* (st1, _) =
     betree_node_apply_messages_fwd self params node_id_cnt (BetreeListCons
-      (key, new_msg) l) st with
-  | Fail e -> Fail e
-  | Return (st1, _) ->
-    begin match
-      betree_node_apply_messages_back'a self params node_id_cnt (BetreeListCons
-        (key, new_msg) l) st st1 with
-    | Fail e -> Fail e
-    | Return (st2, (_, _)) ->
-      begin match
-        betree_node_apply_messages_back1 self params node_id_cnt
-          (BetreeListCons (key, new_msg) l) st st2 with
-      | Fail e -> Fail e
-      | Return _ -> Return (st0, ())
-      end
-    end
-  end
+      (key, new_msg) l) st in
+  let* (st2, (_, _)) =
+    betree_node_apply_messages_back'a self params node_id_cnt (BetreeListCons
+      (key, new_msg) l) st st1 in
+  let* _ =
+    betree_node_apply_messages_back1 self params node_id_cnt (BetreeListCons
+      (key, new_msg) l) st st2 in
+  Return (st0, ())
 
 (** [betree_main::betree::BeTree::{6}::new] *)
 let betree_be_tree_new_fwd
   (min_flush_size : u64) (split_size : u64) (st : state) :
   result (state & betree_be_tree_t)
   =
-  begin match betree_node_id_counter_new_fwd with
-  | Fail e -> Fail e
-  | Return node_id_cnt ->
-    begin match betree_node_id_counter_fresh_id_fwd node_id_cnt with
-    | Fail e -> Fail e
-    | Return id ->
-      begin match betree_store_leaf_node_fwd id BetreeListNil st with
-      | Fail e -> Fail e
-      | Return (st0, _) ->
-        begin match betree_node_id_counter_fresh_id_back node_id_cnt with
-        | Fail e -> Fail e
-        | Return node_id_cnt0 ->
-          Return (st0, Mkbetree_be_tree_t (Mkbetree_params_t min_flush_size
-            split_size) node_id_cnt0 (BetreeNodeLeaf (Mkbetree_leaf_t id 0)))
-        end
-      end
-    end
-  end
+  let* node_id_cnt = betree_node_id_counter_new_fwd in
+  let* id = betree_node_id_counter_fresh_id_fwd node_id_cnt in
+  let* (st0, _) = betree_store_leaf_node_fwd id BetreeListNil st in
+  let* node_id_cnt0 = betree_node_id_counter_fresh_id_back node_id_cnt in
+  Return (st0, Mkbetree_be_tree_t (Mkbetree_params_t min_flush_size split_size)
+    node_id_cnt0 (BetreeNodeLeaf (Mkbetree_leaf_t id 0)))
 
 (** [betree_main::betree::BeTree::{6}::apply] *)
 let betree_be_tree_apply_fwd
   (self : betree_be_tree_t) (key : u64) (msg : betree_message_t) (st : state) :
   result (state & unit)
   =
-  begin match
+  let* (st0, _) =
     betree_node_apply_fwd self.betree_be_tree_root self.betree_be_tree_params
-      self.betree_be_tree_node_id_cnt key msg st with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match
-      betree_node_apply_back'a self.betree_be_tree_root
-        self.betree_be_tree_params self.betree_be_tree_node_id_cnt key msg st
-        st0 with
-    | Fail e -> Fail e
-    | Return (st1, (_, _)) ->
-      betree_node_apply_back1 self.betree_be_tree_root
-        self.betree_be_tree_params self.betree_be_tree_node_id_cnt key msg st
-        st1
-    end
-  end
+      self.betree_be_tree_node_id_cnt key msg st in
+  let* (st1, (_, _)) =
+    betree_node_apply_back'a self.betree_be_tree_root
+      self.betree_be_tree_params self.betree_be_tree_node_id_cnt key msg st st0
+    in
+  betree_node_apply_back1 self.betree_be_tree_root self.betree_be_tree_params
+    self.betree_be_tree_node_id_cnt key msg st st1
 
 (** [betree_main::betree::BeTree::{6}::apply] *)
 let betree_be_tree_apply_back
@@ -1791,44 +1131,28 @@ let betree_be_tree_apply_back
   (st0 : state) :
   result (state & betree_be_tree_t)
   =
-  begin match
+  let* (st1, _) =
     betree_node_apply_fwd self.betree_be_tree_root self.betree_be_tree_params
-      self.betree_be_tree_node_id_cnt key msg st with
-  | Fail e -> Fail e
-  | Return (st1, _) ->
-    begin match
-      betree_node_apply_back'a self.betree_be_tree_root
-        self.betree_be_tree_params self.betree_be_tree_node_id_cnt key msg st
-        st1 with
-    | Fail e -> Fail e
-    | Return (st2, (n, nic)) ->
-      begin match
-        betree_node_apply_back1 self.betree_be_tree_root
-          self.betree_be_tree_params self.betree_be_tree_node_id_cnt key msg st
-          st2 with
-      | Fail e -> Fail e
-      | Return _ ->
-        Return (st0, Mkbetree_be_tree_t self.betree_be_tree_params nic n)
-      end
-    end
-  end
+      self.betree_be_tree_node_id_cnt key msg st in
+  let* (st2, (n, nic)) =
+    betree_node_apply_back'a self.betree_be_tree_root
+      self.betree_be_tree_params self.betree_be_tree_node_id_cnt key msg st st1
+    in
+  let* _ =
+    betree_node_apply_back1 self.betree_be_tree_root self.betree_be_tree_params
+      self.betree_be_tree_node_id_cnt key msg st st2 in
+  Return (st0, Mkbetree_be_tree_t self.betree_be_tree_params nic n)
 
 (** [betree_main::betree::BeTree::{6}::insert] *)
 let betree_be_tree_insert_fwd
   (self : betree_be_tree_t) (key : u64) (value : u64) (st : state) :
   result (state & unit)
   =
-  begin match betree_be_tree_apply_fwd self key (BetreeMessageInsert value) st
-    with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match
-      betree_be_tree_apply_back self key (BetreeMessageInsert value) st st0
-      with
-    | Fail e -> Fail e
-    | Return (st1, _) -> Return (st1, ())
-    end
-  end
+  let* (st0, _) =
+    betree_be_tree_apply_fwd self key (BetreeMessageInsert value) st in
+  let* (st1, _) =
+    betree_be_tree_apply_back self key (BetreeMessageInsert value) st st0 in
+  Return (st1, ())
 
 (** [betree_main::betree::BeTree::{6}::insert] *)
 let betree_be_tree_insert_back
@@ -1836,45 +1160,29 @@ let betree_be_tree_insert_back
   (st0 : state) :
   result (state & betree_be_tree_t)
   =
-  begin match betree_be_tree_apply_fwd self key (BetreeMessageInsert value) st
-    with
-  | Fail e -> Fail e
-  | Return (st1, _) ->
-    begin match
-      betree_be_tree_apply_back self key (BetreeMessageInsert value) st st1
-      with
-    | Fail e -> Fail e
-    | Return (_, self0) -> Return (st0, self0)
-    end
-  end
+  let* (st1, _) =
+    betree_be_tree_apply_fwd self key (BetreeMessageInsert value) st in
+  let* (_, self0) =
+    betree_be_tree_apply_back self key (BetreeMessageInsert value) st st1 in
+  Return (st0, self0)
 
 (** [betree_main::betree::BeTree::{6}::delete] *)
 let betree_be_tree_delete_fwd
   (self : betree_be_tree_t) (key : u64) (st : state) : result (state & unit) =
-  begin match betree_be_tree_apply_fwd self key BetreeMessageDelete st with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match betree_be_tree_apply_back self key BetreeMessageDelete st st0
-      with
-    | Fail e -> Fail e
-    | Return (st1, _) -> Return (st1, ())
-    end
-  end
+  let* (st0, _) = betree_be_tree_apply_fwd self key BetreeMessageDelete st in
+  let* (st1, _) = betree_be_tree_apply_back self key BetreeMessageDelete st st0
+    in
+  Return (st1, ())
 
 (** [betree_main::betree::BeTree::{6}::delete] *)
 let betree_be_tree_delete_back
   (self : betree_be_tree_t) (key : u64) (st : state) (st0 : state) :
   result (state & betree_be_tree_t)
   =
-  begin match betree_be_tree_apply_fwd self key BetreeMessageDelete st with
-  | Fail e -> Fail e
-  | Return (st1, _) ->
-    begin match betree_be_tree_apply_back self key BetreeMessageDelete st st1
-      with
-    | Fail e -> Fail e
-    | Return (_, self0) -> Return (st0, self0)
-    end
-  end
+  let* (st1, _) = betree_be_tree_apply_fwd self key BetreeMessageDelete st in
+  let* (_, self0) =
+    betree_be_tree_apply_back self key BetreeMessageDelete st st1 in
+  Return (st0, self0)
 
 (** [betree_main::betree::BeTree::{6}::upsert] *)
 let betree_be_tree_upsert_fwd
@@ -1882,16 +1190,11 @@ let betree_be_tree_upsert_fwd
   (st : state) :
   result (state & unit)
   =
-  begin match betree_be_tree_apply_fwd self key (BetreeMessageUpsert upd) st
-    with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match
-      betree_be_tree_apply_back self key (BetreeMessageUpsert upd) st st0 with
-    | Fail e -> Fail e
-    | Return (st1, _) -> Return (st1, ())
-    end
-  end
+  let* (st0, _) =
+    betree_be_tree_apply_fwd self key (BetreeMessageUpsert upd) st in
+  let* (st1, _) =
+    betree_be_tree_apply_back self key (BetreeMessageUpsert upd) st st0 in
+  Return (st1, ())
 
 (** [betree_main::betree::BeTree::{6}::upsert] *)
 let betree_be_tree_upsert_back
@@ -1899,16 +1202,11 @@ let betree_be_tree_upsert_back
   (st : state) (st0 : state) :
   result (state & betree_be_tree_t)
   =
-  begin match betree_be_tree_apply_fwd self key (BetreeMessageUpsert upd) st
-    with
-  | Fail e -> Fail e
-  | Return (st1, _) ->
-    begin match
-      betree_be_tree_apply_back self key (BetreeMessageUpsert upd) st st1 with
-    | Fail e -> Fail e
-    | Return (_, self0) -> Return (st0, self0)
-    end
-  end
+  let* (st1, _) =
+    betree_be_tree_apply_fwd self key (BetreeMessageUpsert upd) st in
+  let* (_, self0) =
+    betree_be_tree_apply_back self key (BetreeMessageUpsert upd) st st1 in
+  Return (st0, self0)
 
 (** [betree_main::betree::BeTree::{6}::lookup] *)
 let betree_be_tree_lookup_fwd
@@ -1922,12 +1220,10 @@ let betree_be_tree_lookup_back
   (self : betree_be_tree_t) (key : u64) (st : state) (st0 : state) :
   result (state & betree_be_tree_t)
   =
-  begin match betree_node_lookup_back self.betree_be_tree_root key st st0 with
-  | Fail e -> Fail e
-  | Return (st1, n) ->
-    Return (st1, Mkbetree_be_tree_t self.betree_be_tree_params
-      self.betree_be_tree_node_id_cnt n)
-  end
+  let* (st1, n) = betree_node_lookup_back self.betree_be_tree_root key st st0
+    in
+  Return (st1, Mkbetree_be_tree_t self.betree_be_tree_params
+    self.betree_be_tree_node_id_cnt n)
 
 (** [betree_main::main] *)
 let main_fwd : result unit = Return ()
diff --git a/tests/fstar/betree_back_stateful/Primitives.fst b/tests/fstar/betree_back_stateful/Primitives.fst
index bf0f9078..98a696b6 100644
--- a/tests/fstar/betree_back_stateful/Primitives.fst
+++ b/tests/fstar/betree_back_stateful/Primitives.fst
@@ -35,7 +35,9 @@ unfold let return (#a : Type0) (x : a) : result a = Return x
 // let* x = y in
 // ...
 // ```
-unfold let (let*) (#a #b : Type0) (m: result a) (f: a -> result b) : result b =
+unfold let (let*) (#a #b : Type0) (m: result a)
+  (f: (x:a) -> Pure (result b) (requires (m == Return x)) (ensures fun _ -> True)) :
+  result b =
   match m with
   | Return x -> f x
   | Fail e   -> Fail e
diff --git a/tests/fstar/hashmap/Hashmap.Funs.fst b/tests/fstar/hashmap/Hashmap.Funs.fst
index 68bda221..0140aadc 100644
--- a/tests/fstar/hashmap/Hashmap.Funs.fst
+++ b/tests/fstar/hashmap/Hashmap.Funs.fst
@@ -17,15 +17,10 @@ let rec hash_map_allocate_slots_loop_fwd
   (decreases (hash_map_allocate_slots_loop_decreases t slots n))
   =
   if n > 0
-  then
-    begin match vec_push_back (list_t t) slots ListNil with
-    | Fail e -> Fail e
-    | Return slots0 ->
-      begin match usize_sub n 1 with
-      | Fail e -> Fail e
-      | Return n0 -> hash_map_allocate_slots_loop_fwd t slots0 n0
-      end
-    end
+  then begin
+    let* slots0 = vec_push_back (list_t t) slots ListNil in
+    let* n0 = usize_sub n 1 in
+    hash_map_allocate_slots_loop_fwd t slots0 n0 end
   else Return slots
 
 (** [hashmap::HashMap::{0}::allocate_slots] *)
@@ -40,19 +35,10 @@ let hash_map_new_with_capacity_fwd
   result (hash_map_t t)
   =
   let v = vec_new (list_t t) in
-  begin match hash_map_allocate_slots_fwd t v capacity with
-  | Fail e -> Fail e
-  | Return slots ->
-    begin match usize_mul capacity max_load_dividend with
-    | Fail e -> Fail e
-    | Return i ->
-      begin match usize_div i max_load_divisor with
-      | Fail e -> Fail e
-      | Return i0 ->
-        Return (Mkhash_map_t 0 (max_load_dividend, max_load_divisor) i0 slots)
-      end
-    end
-  end
+  let* slots = hash_map_allocate_slots_fwd t v capacity in
+  let* i = usize_mul capacity max_load_dividend in
+  let* i0 = usize_div i max_load_divisor in
+  Return (Mkhash_map_t 0 (max_load_dividend, max_load_divisor) i0 slots)
 
 (** [hashmap::HashMap::{0}::new] *)
 let hash_map_new_fwd (t : Type0) : result (hash_map_t t) =
@@ -66,26 +52,18 @@ let rec hash_map_clear_loop_fwd_back
   =
   let i0 = vec_len (list_t t) slots in
   if i < i0
-  then
-    begin match usize_add i 1 with
-    | Fail e -> Fail e
-    | Return i1 ->
-      begin match vec_index_mut_back (list_t t) slots i ListNil with
-      | Fail e -> Fail e
-      | Return slots0 -> hash_map_clear_loop_fwd_back t slots0 i1
-      end
-    end
+  then begin
+    let* i1 = usize_add i 1 in
+    let* slots0 = vec_index_mut_back (list_t t) slots i ListNil in
+    hash_map_clear_loop_fwd_back t slots0 i1 end
   else Return slots
 
 (** [hashmap::HashMap::{0}::clear] *)
 let hash_map_clear_fwd_back
   (t : Type0) (self : hash_map_t t) : result (hash_map_t t) =
-  begin match hash_map_clear_loop_fwd_back t self.hash_map_slots 0 with
-  | Fail e -> Fail e
-  | Return v ->
-    Return (Mkhash_map_t 0 self.hash_map_max_load_factor self.hash_map_max_load
-      v)
-  end
+  let* v = hash_map_clear_loop_fwd_back t self.hash_map_slots 0 in
+  Return (Mkhash_map_t 0 self.hash_map_max_load_factor self.hash_map_max_load
+    v)
 
 (** [hashmap::HashMap::{0}::len] *)
 let hash_map_len_fwd (t : Type0) (self : hash_map_t t) : result usize =
@@ -120,11 +98,9 @@ let rec hash_map_insert_in_list_loop_back
   | ListCons ckey cvalue tl ->
     if ckey = key
     then Return (ListCons ckey value tl)
-    else
-      begin match hash_map_insert_in_list_loop_back t key value tl with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (ListCons ckey cvalue tl0)
-      end
+    else begin
+      let* tl0 = hash_map_insert_in_list_loop_back t key value tl in
+      Return (ListCons ckey cvalue tl0) end
   | ListNil -> let l = ListNil in Return (ListCons key value l)
   end
 
@@ -138,55 +114,23 @@ let hash_map_insert_no_resize_fwd_back
   (t : Type0) (self : hash_map_t t) (key : usize) (value : t) :
   result (hash_map_t t)
   =
-  begin match hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (list_t t) self.hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod
-        with
-      | Fail e -> Fail e
-      | Return l ->
-        begin match hash_map_insert_in_list_fwd t key value l with
-        | Fail e -> Fail e
-        | Return inserted ->
-          if inserted
-          then
-            begin match usize_add self.hash_map_num_entries 1 with
-            | Fail e -> Fail e
-            | Return i0 ->
-              begin match hash_map_insert_in_list_back t key value l with
-              | Fail e -> Fail e
-              | Return l0 ->
-                begin match
-                  vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0
-                  with
-                | Fail e -> Fail e
-                | Return v ->
-                  Return (Mkhash_map_t i0 self.hash_map_max_load_factor
-                    self.hash_map_max_load v)
-                end
-              end
-            end
-          else
-            begin match hash_map_insert_in_list_back t key value l with
-            | Fail e -> Fail e
-            | Return l0 ->
-              begin match
-                vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0
-                with
-              | Fail e -> Fail e
-              | Return v ->
-                Return (Mkhash_map_t self.hash_map_num_entries
-                  self.hash_map_max_load_factor self.hash_map_max_load v)
-              end
-            end
-        end
-      end
-    end
-  end
+  let* hash = hash_key_fwd key in
+  let i = vec_len (list_t t) self.hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l = vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod in
+  let* inserted = hash_map_insert_in_list_fwd t key value l in
+  if inserted
+  then begin
+    let* i0 = usize_add self.hash_map_num_entries 1 in
+    let* l0 = hash_map_insert_in_list_back t key value l in
+    let* v = vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0 in
+    Return (Mkhash_map_t i0 self.hash_map_max_load_factor
+      self.hash_map_max_load v) end
+  else begin
+    let* l0 = hash_map_insert_in_list_back t key value l in
+    let* v = vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0 in
+    Return (Mkhash_map_t self.hash_map_num_entries
+      self.hash_map_max_load_factor self.hash_map_max_load v) end
 
 (** [core::num::u32::{9}::MAX] *)
 let core_num_u32_max_body : result u32 = Return 4294967295
@@ -200,11 +144,8 @@ let rec hash_map_move_elements_from_list_loop_fwd_back
   =
   begin match ls with
   | ListCons k v tl ->
-    begin match hash_map_insert_no_resize_fwd_back t ntable k v with
-    | Fail e -> Fail e
-    | Return ntable0 ->
-      hash_map_move_elements_from_list_loop_fwd_back t ntable0 tl
-    end
+    let* ntable0 = hash_map_insert_no_resize_fwd_back t ntable k v in
+    hash_map_move_elements_from_list_loop_fwd_back t ntable0 tl
   | ListNil -> Return ntable
   end
 
@@ -221,26 +162,14 @@ let rec hash_map_move_elements_loop_fwd_back
   =
   let i0 = vec_len (list_t t) slots in
   if i < i0
-  then
-    begin match vec_index_mut_fwd (list_t t) slots i with
-    | Fail e -> Fail e
-    | Return l ->
-      let ls = mem_replace_fwd (list_t t) l ListNil in
-      begin match hash_map_move_elements_from_list_fwd_back t ntable ls with
-      | Fail e -> Fail e
-      | Return ntable0 ->
-        begin match usize_add i 1 with
-        | Fail e -> Fail e
-        | Return i1 ->
-          let l0 = mem_replace_back (list_t t) l ListNil in
-          begin match vec_index_mut_back (list_t t) slots i l0 with
-          | Fail e -> Fail e
-          | Return slots0 ->
-            hash_map_move_elements_loop_fwd_back t ntable0 slots0 i1
-          end
-        end
-      end
-    end
+  then begin
+    let* l = vec_index_mut_fwd (list_t t) slots i in
+    let ls = mem_replace_fwd (list_t t) l ListNil in
+    let* ntable0 = hash_map_move_elements_from_list_fwd_back t ntable ls in
+    let* i1 = usize_add i 1 in
+    let l0 = mem_replace_back (list_t t) l ListNil in
+    let* slots0 = vec_index_mut_back (list_t t) slots i l0 in
+    hash_map_move_elements_loop_fwd_back t ntable0 slots0 i1 end
   else Return (ntable, slots)
 
 (** [hashmap::HashMap::{0}::move_elements] *)
@@ -253,58 +182,33 @@ let hash_map_move_elements_fwd_back
 (** [hashmap::HashMap::{0}::try_resize] *)
 let hash_map_try_resize_fwd_back
   (t : Type0) (self : hash_map_t t) : result (hash_map_t t) =
-  begin match scalar_cast U32 Usize core_num_u32_max_c with
-  | Fail e -> Fail e
-  | Return max_usize ->
-    let capacity = vec_len (list_t t) self.hash_map_slots in
-    begin match usize_div max_usize 2 with
-    | Fail e -> Fail e
-    | Return n1 ->
-      let (i, i0) = self.hash_map_max_load_factor in
-      begin match usize_div n1 i with
-      | Fail e -> Fail e
-      | Return i1 ->
-        if capacity <= i1
-        then
-          begin match usize_mul capacity 2 with
-          | Fail e -> Fail e
-          | Return i2 ->
-            begin match hash_map_new_with_capacity_fwd t i2 i i0 with
-            | Fail e -> Fail e
-            | Return ntable ->
-              begin match
-                hash_map_move_elements_fwd_back t ntable self.hash_map_slots 0
-                with
-              | Fail e -> Fail e
-              | Return (ntable0, _) ->
-                Return (Mkhash_map_t self.hash_map_num_entries (i, i0)
-                  ntable0.hash_map_max_load ntable0.hash_map_slots)
-              end
-            end
-          end
-        else
-          Return (Mkhash_map_t self.hash_map_num_entries (i, i0)
-            self.hash_map_max_load self.hash_map_slots)
-      end
-    end
-  end
+  let* max_usize = scalar_cast U32 Usize core_num_u32_max_c in
+  let capacity = vec_len (list_t t) self.hash_map_slots in
+  let* n1 = usize_div max_usize 2 in
+  let (i, i0) = self.hash_map_max_load_factor in
+  let* i1 = usize_div n1 i in
+  if capacity <= i1
+  then begin
+    let* i2 = usize_mul capacity 2 in
+    let* ntable = hash_map_new_with_capacity_fwd t i2 i i0 in
+    let* (ntable0, _) =
+      hash_map_move_elements_fwd_back t ntable self.hash_map_slots 0 in
+    Return (Mkhash_map_t self.hash_map_num_entries (i, i0)
+      ntable0.hash_map_max_load ntable0.hash_map_slots) end
+  else
+    Return (Mkhash_map_t self.hash_map_num_entries (i, i0)
+      self.hash_map_max_load self.hash_map_slots)
 
 (** [hashmap::HashMap::{0}::insert] *)
 let hash_map_insert_fwd_back
   (t : Type0) (self : hash_map_t t) (key : usize) (value : t) :
   result (hash_map_t t)
   =
-  begin match hash_map_insert_no_resize_fwd_back t self key value with
-  | Fail e -> Fail e
-  | Return self0 ->
-    begin match hash_map_len_fwd t self0 with
-    | Fail e -> Fail e
-    | Return i ->
-      if i > self0.hash_map_max_load
-      then hash_map_try_resize_fwd_back t self0
-      else Return self0
-    end
-  end
+  let* self0 = hash_map_insert_no_resize_fwd_back t self key value in
+  let* i = hash_map_len_fwd t self0 in
+  if i > self0.hash_map_max_load
+  then hash_map_try_resize_fwd_back t self0
+  else Return self0
 
 (** [hashmap::HashMap::{0}::contains_key_in_list] *)
 let rec hash_map_contains_key_in_list_loop_fwd
@@ -328,19 +232,11 @@ let hash_map_contains_key_in_list_fwd
 (** [hashmap::HashMap::{0}::contains_key] *)
 let hash_map_contains_key_fwd
   (t : Type0) (self : hash_map_t t) (key : usize) : result bool =
-  begin match hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (list_t t) self.hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match vec_index_fwd (list_t t) self.hash_map_slots hash_mod with
-      | Fail e -> Fail e
-      | Return l -> hash_map_contains_key_in_list_fwd t key l
-      end
-    end
-  end
+  let* hash = hash_key_fwd key in
+  let i = vec_len (list_t t) self.hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l = vec_index_fwd (list_t t) self.hash_map_slots hash_mod in
+  hash_map_contains_key_in_list_fwd t key l
 
 (** [hashmap::HashMap::{0}::get_in_list] *)
 let rec hash_map_get_in_list_loop_fwd
@@ -363,19 +259,11 @@ let hash_map_get_in_list_fwd
 (** [hashmap::HashMap::{0}::get] *)
 let hash_map_get_fwd
   (t : Type0) (self : hash_map_t t) (key : usize) : result t =
-  begin match hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (list_t t) self.hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match vec_index_fwd (list_t t) self.hash_map_slots hash_mod with
-      | Fail e -> Fail e
-      | Return l -> hash_map_get_in_list_fwd t key l
-      end
-    end
-  end
+  let* hash = hash_key_fwd key in
+  let i = vec_len (list_t t) self.hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l = vec_index_fwd (list_t t) self.hash_map_slots hash_mod in
+  hash_map_get_in_list_fwd t key l
 
 (** [hashmap::HashMap::{0}::get_mut_in_list] *)
 let rec hash_map_get_mut_in_list_loop_fwd
@@ -405,11 +293,9 @@ let rec hash_map_get_mut_in_list_loop_back
   | ListCons ckey cvalue tl ->
     if ckey = key
     then Return (ListCons ckey ret tl)
-    else
-      begin match hash_map_get_mut_in_list_loop_back t tl key ret with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (ListCons ckey cvalue tl0)
-      end
+    else begin
+      let* tl0 = hash_map_get_mut_in_list_loop_back t tl key ret in
+      Return (ListCons ckey cvalue tl0) end
   | ListNil -> Fail Failure
   end
 
@@ -421,51 +307,25 @@ let hash_map_get_mut_in_list_back
 (** [hashmap::HashMap::{0}::get_mut] *)
 let hash_map_get_mut_fwd
   (t : Type0) (self : hash_map_t t) (key : usize) : result t =
-  begin match hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (list_t t) self.hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod
-        with
-      | Fail e -> Fail e
-      | Return l -> hash_map_get_mut_in_list_fwd t l key
-      end
-    end
-  end
+  let* hash = hash_key_fwd key in
+  let i = vec_len (list_t t) self.hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l = vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod in
+  hash_map_get_mut_in_list_fwd t l key
 
 (** [hashmap::HashMap::{0}::get_mut] *)
 let hash_map_get_mut_back
   (t : Type0) (self : hash_map_t t) (key : usize) (ret : t) :
   result (hash_map_t t)
   =
-  begin match hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (list_t t) self.hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod
-        with
-      | Fail e -> Fail e
-      | Return l ->
-        begin match hash_map_get_mut_in_list_back t l key ret with
-        | Fail e -> Fail e
-        | Return l0 ->
-          begin match
-            vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0 with
-          | Fail e -> Fail e
-          | Return v ->
-            Return (Mkhash_map_t self.hash_map_num_entries
-              self.hash_map_max_load_factor self.hash_map_max_load v)
-          end
-        end
-      end
-    end
-  end
+  let* hash = hash_key_fwd key in
+  let i = vec_len (list_t t) self.hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l = vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod in
+  let* l0 = hash_map_get_mut_in_list_back t l key ret in
+  let* v = vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0 in
+  Return (Mkhash_map_t self.hash_map_num_entries self.hash_map_max_load_factor
+    self.hash_map_max_load v)
 
 (** [hashmap::HashMap::{0}::remove_from_list] *)
 let rec hash_map_remove_from_list_loop_fwd
@@ -506,11 +366,9 @@ let rec hash_map_remove_from_list_loop_back
       | ListCons i cvalue tl0 -> Return tl0
       | ListNil -> Fail Failure
       end
-    else
-      begin match hash_map_remove_from_list_loop_back t key tl with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (ListCons ckey x tl0)
-      end
+    else begin
+      let* tl0 = hash_map_remove_from_list_loop_back t key tl in
+      Return (ListCons ckey x tl0) end
   | ListNil -> Return ListNil
   end
 
@@ -522,164 +380,74 @@ let hash_map_remove_from_list_back
 (** [hashmap::HashMap::{0}::remove] *)
 let hash_map_remove_fwd
   (t : Type0) (self : hash_map_t t) (key : usize) : result (option t) =
-  begin match hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (list_t t) self.hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod
-        with
-      | Fail e -> Fail e
-      | Return l ->
-        begin match hash_map_remove_from_list_fwd t key l with
-        | Fail e -> Fail e
-        | Return x ->
-          begin match x with
-          | None -> Return None
-          | Some x0 ->
-            begin match usize_sub self.hash_map_num_entries 1 with
-            | Fail e -> Fail e
-            | Return _ -> Return (Some x0)
-            end
-          end
-        end
-      end
-    end
+  let* hash = hash_key_fwd key in
+  let i = vec_len (list_t t) self.hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l = vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod in
+  let* x = hash_map_remove_from_list_fwd t key l in
+  begin match x with
+  | None -> Return None
+  | Some x0 ->
+    let* _ = usize_sub self.hash_map_num_entries 1 in Return (Some x0)
   end
 
 (** [hashmap::HashMap::{0}::remove] *)
 let hash_map_remove_back
   (t : Type0) (self : hash_map_t t) (key : usize) : result (hash_map_t t) =
-  begin match hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (list_t t) self.hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod
-        with
-      | Fail e -> Fail e
-      | Return l ->
-        begin match hash_map_remove_from_list_fwd t key l with
-        | Fail e -> Fail e
-        | Return x ->
-          begin match x with
-          | None ->
-            begin match hash_map_remove_from_list_back t key l with
-            | Fail e -> Fail e
-            | Return l0 ->
-              begin match
-                vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0
-                with
-              | Fail e -> Fail e
-              | Return v ->
-                Return (Mkhash_map_t self.hash_map_num_entries
-                  self.hash_map_max_load_factor self.hash_map_max_load v)
-              end
-            end
-          | Some x0 ->
-            begin match usize_sub self.hash_map_num_entries 1 with
-            | Fail e -> Fail e
-            | Return i0 ->
-              begin match hash_map_remove_from_list_back t key l with
-              | Fail e -> Fail e
-              | Return l0 ->
-                begin match
-                  vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0
-                  with
-                | Fail e -> Fail e
-                | Return v ->
-                  Return (Mkhash_map_t i0 self.hash_map_max_load_factor
-                    self.hash_map_max_load v)
-                end
-              end
-            end
-          end
-        end
-      end
-    end
+  let* hash = hash_key_fwd key in
+  let i = vec_len (list_t t) self.hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l = vec_index_mut_fwd (list_t t) self.hash_map_slots hash_mod in
+  let* x = hash_map_remove_from_list_fwd t key l in
+  begin match x with
+  | None ->
+    let* l0 = hash_map_remove_from_list_back t key l in
+    let* v = vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0 in
+    Return (Mkhash_map_t self.hash_map_num_entries
+      self.hash_map_max_load_factor self.hash_map_max_load v)
+  | Some x0 ->
+    let* i0 = usize_sub self.hash_map_num_entries 1 in
+    let* l0 = hash_map_remove_from_list_back t key l in
+    let* v = vec_index_mut_back (list_t t) self.hash_map_slots hash_mod l0 in
+    Return (Mkhash_map_t i0 self.hash_map_max_load_factor
+      self.hash_map_max_load v)
   end
 
 (** [hashmap::test1] *)
 let test1_fwd : result unit =
-  begin match hash_map_new_fwd u64 with
-  | Fail e -> Fail e
-  | Return hm ->
-    begin match hash_map_insert_fwd_back u64 hm 0 42 with
-    | Fail e -> Fail e
-    | Return hm0 ->
-      begin match hash_map_insert_fwd_back u64 hm0 128 18 with
-      | Fail e -> Fail e
-      | Return hm1 ->
-        begin match hash_map_insert_fwd_back u64 hm1 1024 138 with
-        | Fail e -> Fail e
-        | Return hm2 ->
-          begin match hash_map_insert_fwd_back u64 hm2 1056 256 with
-          | Fail e -> Fail e
-          | Return hm3 ->
-            begin match hash_map_get_fwd u64 hm3 128 with
-            | Fail e -> Fail e
-            | Return i ->
-              if not (i = 18)
-              then Fail Failure
-              else
-                begin match hash_map_get_mut_back u64 hm3 1024 56 with
-                | Fail e -> Fail e
-                | Return hm4 ->
-                  begin match hash_map_get_fwd u64 hm4 1024 with
-                  | Fail e -> Fail e
-                  | Return i0 ->
-                    if not (i0 = 56)
-                    then Fail Failure
-                    else
-                      begin match hash_map_remove_fwd u64 hm4 1024 with
-                      | Fail e -> Fail e
-                      | Return x ->
-                        begin match x with
-                        | None -> Fail Failure
-                        | Some x0 ->
-                          if not (x0 = 56)
-                          then Fail Failure
-                          else
-                            begin match hash_map_remove_back u64 hm4 1024 with
-                            | Fail e -> Fail e
-                            | Return hm5 ->
-                              begin match hash_map_get_fwd u64 hm5 0 with
-                              | Fail e -> Fail e
-                              | Return i1 ->
-                                if not (i1 = 42)
-                                then Fail Failure
-                                else
-                                  begin match hash_map_get_fwd u64 hm5 128 with
-                                  | Fail e -> Fail e
-                                  | Return i2 ->
-                                    if not (i2 = 18)
-                                    then Fail Failure
-                                    else
-                                      begin match hash_map_get_fwd u64 hm5 1056
-                                        with
-                                      | Fail e -> Fail e
-                                      | Return i3 ->
-                                        if not (i3 = 256)
-                                        then Fail Failure
-                                        else Return ()
-                                      end
-                                  end
-                              end
-                            end
-                        end
-                      end
-                  end
-                end
-            end
-          end
-        end
-      end
-    end
-  end
+  let* hm = hash_map_new_fwd u64 in
+  let* hm0 = hash_map_insert_fwd_back u64 hm 0 42 in
+  let* hm1 = hash_map_insert_fwd_back u64 hm0 128 18 in
+  let* hm2 = hash_map_insert_fwd_back u64 hm1 1024 138 in
+  let* hm3 = hash_map_insert_fwd_back u64 hm2 1056 256 in
+  let* i = hash_map_get_fwd u64 hm3 128 in
+  if not (i = 18)
+  then Fail Failure
+  else begin
+    let* hm4 = hash_map_get_mut_back u64 hm3 1024 56 in
+    let* i0 = hash_map_get_fwd u64 hm4 1024 in
+    if not (i0 = 56)
+    then Fail Failure
+    else begin
+      let* x = hash_map_remove_fwd u64 hm4 1024 in
+      begin match x with
+      | None -> Fail Failure
+      | Some x0 ->
+        if not (x0 = 56)
+        then Fail Failure
+        else begin
+          let* hm5 = hash_map_remove_back u64 hm4 1024 in
+          let* i1 = hash_map_get_fwd u64 hm5 0 in
+          if not (i1 = 42)
+          then Fail Failure
+          else begin
+            let* i2 = hash_map_get_fwd u64 hm5 128 in
+            if not (i2 = 18)
+            then Fail Failure
+            else begin
+              let* i3 = hash_map_get_fwd u64 hm5 1056 in
+              if not (i3 = 256) then Fail Failure else Return () end end end
+      end end end
 
 (** Unit test for [hashmap::test1] *)
 let _ = assert_norm (test1_fwd = Return ())
diff --git a/tests/fstar/hashmap/Primitives.fst b/tests/fstar/hashmap/Primitives.fst
index bf0f9078..98a696b6 100644
--- a/tests/fstar/hashmap/Primitives.fst
+++ b/tests/fstar/hashmap/Primitives.fst
@@ -35,7 +35,9 @@ unfold let return (#a : Type0) (x : a) : result a = Return x
 // let* x = y in
 // ...
 // ```
-unfold let (let*) (#a #b : Type0) (m: result a) (f: a -> result b) : result b =
+unfold let (let*) (#a #b : Type0) (m: result a)
+  (f: (x:a) -> Pure (result b) (requires (m == Return x)) (ensures fun _ -> True)) :
+  result b =
   match m with
   | Return x -> f x
   | Fail e   -> Fail e
diff --git a/tests/fstar/hashmap_on_disk/HashmapMain.Funs.fst b/tests/fstar/hashmap_on_disk/HashmapMain.Funs.fst
index 56cff453..51021daf 100644
--- a/tests/fstar/hashmap_on_disk/HashmapMain.Funs.fst
+++ b/tests/fstar/hashmap_on_disk/HashmapMain.Funs.fst
@@ -18,15 +18,10 @@ let rec hashmap_hash_map_allocate_slots_loop_fwd
   (decreases (hashmap_hash_map_allocate_slots_loop_decreases t slots n))
   =
   if n > 0
-  then
-    begin match vec_push_back (hashmap_list_t t) slots HashmapListNil with
-    | Fail e -> Fail e
-    | Return slots0 ->
-      begin match usize_sub n 1 with
-      | Fail e -> Fail e
-      | Return n0 -> hashmap_hash_map_allocate_slots_loop_fwd t slots0 n0
-      end
-    end
+  then begin
+    let* slots0 = vec_push_back (hashmap_list_t t) slots HashmapListNil in
+    let* n0 = usize_sub n 1 in
+    hashmap_hash_map_allocate_slots_loop_fwd t slots0 n0 end
   else Return slots
 
 (** [hashmap_main::hashmap::HashMap::{0}::allocate_slots] *)
@@ -43,20 +38,11 @@ let hashmap_hash_map_new_with_capacity_fwd
   result (hashmap_hash_map_t t)
   =
   let v = vec_new (hashmap_list_t t) in
-  begin match hashmap_hash_map_allocate_slots_fwd t v capacity with
-  | Fail e -> Fail e
-  | Return slots ->
-    begin match usize_mul capacity max_load_dividend with
-    | Fail e -> Fail e
-    | Return i ->
-      begin match usize_div i max_load_divisor with
-      | Fail e -> Fail e
-      | Return i0 ->
-        Return (Mkhashmap_hash_map_t 0 (max_load_dividend, max_load_divisor) i0
-          slots)
-      end
-    end
-  end
+  let* slots = hashmap_hash_map_allocate_slots_fwd t v capacity in
+  let* i = usize_mul capacity max_load_dividend in
+  let* i0 = usize_div i max_load_divisor in
+  Return (Mkhashmap_hash_map_t 0 (max_load_dividend, max_load_divisor) i0
+    slots)
 
 (** [hashmap_main::hashmap::HashMap::{0}::new] *)
 let hashmap_hash_map_new_fwd (t : Type0) : result (hashmap_hash_map_t t) =
@@ -70,28 +56,20 @@ let rec hashmap_hash_map_clear_loop_fwd_back
   =
   let i0 = vec_len (hashmap_list_t t) slots in
   if i < i0
-  then
-    begin match usize_add i 1 with
-    | Fail e -> Fail e
-    | Return i1 ->
-      begin match vec_index_mut_back (hashmap_list_t t) slots i HashmapListNil
-        with
-      | Fail e -> Fail e
-      | Return slots0 -> hashmap_hash_map_clear_loop_fwd_back t slots0 i1
-      end
-    end
+  then begin
+    let* i1 = usize_add i 1 in
+    let* slots0 = vec_index_mut_back (hashmap_list_t t) slots i HashmapListNil
+      in
+    hashmap_hash_map_clear_loop_fwd_back t slots0 i1 end
   else Return slots
 
 (** [hashmap_main::hashmap::HashMap::{0}::clear] *)
 let hashmap_hash_map_clear_fwd_back
   (t : Type0) (self : hashmap_hash_map_t t) : result (hashmap_hash_map_t t) =
-  begin match
-    hashmap_hash_map_clear_loop_fwd_back t self.hashmap_hash_map_slots 0 with
-  | Fail e -> Fail e
-  | Return v ->
-    Return (Mkhashmap_hash_map_t 0 self.hashmap_hash_map_max_load_factor
-      self.hashmap_hash_map_max_load v)
-  end
+  let* v = hashmap_hash_map_clear_loop_fwd_back t self.hashmap_hash_map_slots 0
+    in
+  Return (Mkhashmap_hash_map_t 0 self.hashmap_hash_map_max_load_factor
+    self.hashmap_hash_map_max_load v)
 
 (** [hashmap_main::hashmap::HashMap::{0}::len] *)
 let hashmap_hash_map_len_fwd
@@ -127,11 +105,9 @@ let rec hashmap_hash_map_insert_in_list_loop_back
   | HashmapListCons ckey cvalue tl ->
     if ckey = key
     then Return (HashmapListCons ckey value tl)
-    else
-      begin match hashmap_hash_map_insert_in_list_loop_back t key value tl with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (HashmapListCons ckey cvalue tl0)
-      end
+    else begin
+      let* tl0 = hashmap_hash_map_insert_in_list_loop_back t key value tl in
+      Return (HashmapListCons ckey cvalue tl0) end
   | HashmapListNil ->
     let l = HashmapListNil in Return (HashmapListCons key value l)
   end
@@ -148,59 +124,30 @@ let hashmap_hash_map_insert_no_resize_fwd_back
   (t : Type0) (self : hashmap_hash_map_t t) (key : usize) (value : t) :
   result (hashmap_hash_map_t t)
   =
-  begin match hashmap_hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match
-        vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots
-          hash_mod with
-      | Fail e -> Fail e
-      | Return l ->
-        begin match hashmap_hash_map_insert_in_list_fwd t key value l with
-        | Fail e -> Fail e
-        | Return inserted ->
-          if inserted
-          then
-            begin match usize_add self.hashmap_hash_map_num_entries 1 with
-            | Fail e -> Fail e
-            | Return i0 ->
-              begin match hashmap_hash_map_insert_in_list_back t key value l
-                with
-              | Fail e -> Fail e
-              | Return l0 ->
-                begin match
-                  vec_index_mut_back (hashmap_list_t t)
-                    self.hashmap_hash_map_slots hash_mod l0 with
-                | Fail e -> Fail e
-                | Return v ->
-                  Return (Mkhashmap_hash_map_t i0
-                    self.hashmap_hash_map_max_load_factor
-                    self.hashmap_hash_map_max_load v)
-                end
-              end
-            end
-          else
-            begin match hashmap_hash_map_insert_in_list_back t key value l with
-            | Fail e -> Fail e
-            | Return l0 ->
-              begin match
-                vec_index_mut_back (hashmap_list_t t)
-                  self.hashmap_hash_map_slots hash_mod l0 with
-              | Fail e -> Fail e
-              | Return v ->
-                Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries
-                  self.hashmap_hash_map_max_load_factor
-                  self.hashmap_hash_map_max_load v)
-              end
-            end
-        end
-      end
+  let* hash = hashmap_hash_key_fwd key in
+  let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l =
+    vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod
+    in
+  let* inserted = hashmap_hash_map_insert_in_list_fwd t key value l in
+  if inserted
+  then begin
+    let* i0 = usize_add self.hashmap_hash_map_num_entries 1 in
+    let* l0 = hashmap_hash_map_insert_in_list_back t key value l in
+    let* v =
+      vec_index_mut_back (hashmap_list_t t) self.hashmap_hash_map_slots
+        hash_mod l0 in
+    Return (Mkhashmap_hash_map_t i0 self.hashmap_hash_map_max_load_factor
+      self.hashmap_hash_map_max_load v) end
+  else begin
+    let* l0 = hashmap_hash_map_insert_in_list_back t key value l in
+    let* v =
+      vec_index_mut_back (hashmap_list_t t) self.hashmap_hash_map_slots
+        hash_mod l0 in
+    Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries
+      self.hashmap_hash_map_max_load_factor self.hashmap_hash_map_max_load v)
     end
-  end
 
 (** [core::num::u32::{9}::MAX] *)
 let core_num_u32_max_body : result u32 = Return 4294967295
@@ -215,11 +162,8 @@ let rec hashmap_hash_map_move_elements_from_list_loop_fwd_back
   =
   begin match ls with
   | HashmapListCons k v tl ->
-    begin match hashmap_hash_map_insert_no_resize_fwd_back t ntable k v with
-    | Fail e -> Fail e
-    | Return ntable0 ->
-      hashmap_hash_map_move_elements_from_list_loop_fwd_back t ntable0 tl
-    end
+    let* ntable0 = hashmap_hash_map_insert_no_resize_fwd_back t ntable k v in
+    hashmap_hash_map_move_elements_from_list_loop_fwd_back t ntable0 tl
   | HashmapListNil -> Return ntable
   end
 
@@ -239,27 +183,15 @@ let rec hashmap_hash_map_move_elements_loop_fwd_back
   =
   let i0 = vec_len (hashmap_list_t t) slots in
   if i < i0
-  then
-    begin match vec_index_mut_fwd (hashmap_list_t t) slots i with
-    | Fail e -> Fail e
-    | Return l ->
-      let ls = mem_replace_fwd (hashmap_list_t t) l HashmapListNil in
-      begin match hashmap_hash_map_move_elements_from_list_fwd_back t ntable ls
-        with
-      | Fail e -> Fail e
-      | Return ntable0 ->
-        begin match usize_add i 1 with
-        | Fail e -> Fail e
-        | Return i1 ->
-          let l0 = mem_replace_back (hashmap_list_t t) l HashmapListNil in
-          begin match vec_index_mut_back (hashmap_list_t t) slots i l0 with
-          | Fail e -> Fail e
-          | Return slots0 ->
-            hashmap_hash_map_move_elements_loop_fwd_back t ntable0 slots0 i1
-          end
-        end
-      end
-    end
+  then begin
+    let* l = vec_index_mut_fwd (hashmap_list_t t) slots i in
+    let ls = mem_replace_fwd (hashmap_list_t t) l HashmapListNil in
+    let* ntable0 =
+      hashmap_hash_map_move_elements_from_list_fwd_back t ntable ls in
+    let* i1 = usize_add i 1 in
+    let l0 = mem_replace_back (hashmap_list_t t) l HashmapListNil in
+    let* slots0 = vec_index_mut_back (hashmap_list_t t) slots i l0 in
+    hashmap_hash_map_move_elements_loop_fwd_back t ntable0 slots0 i1 end
   else Return (ntable, slots)
 
 (** [hashmap_main::hashmap::HashMap::{0}::move_elements] *)
@@ -273,59 +205,34 @@ let hashmap_hash_map_move_elements_fwd_back
 (** [hashmap_main::hashmap::HashMap::{0}::try_resize] *)
 let hashmap_hash_map_try_resize_fwd_back
   (t : Type0) (self : hashmap_hash_map_t t) : result (hashmap_hash_map_t t) =
-  begin match scalar_cast U32 Usize core_num_u32_max_c with
-  | Fail e -> Fail e
-  | Return max_usize ->
-    let capacity = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
-    begin match usize_div max_usize 2 with
-    | Fail e -> Fail e
-    | Return n1 ->
-      let (i, i0) = self.hashmap_hash_map_max_load_factor in
-      begin match usize_div n1 i with
-      | Fail e -> Fail e
-      | Return i1 ->
-        if capacity <= i1
-        then
-          begin match usize_mul capacity 2 with
-          | Fail e -> Fail e
-          | Return i2 ->
-            begin match hashmap_hash_map_new_with_capacity_fwd t i2 i i0 with
-            | Fail e -> Fail e
-            | Return ntable ->
-              begin match
-                hashmap_hash_map_move_elements_fwd_back t ntable
-                  self.hashmap_hash_map_slots 0 with
-              | Fail e -> Fail e
-              | Return (ntable0, _) ->
-                Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries
-                  (i, i0) ntable0.hashmap_hash_map_max_load
-                  ntable0.hashmap_hash_map_slots)
-              end
-            end
-          end
-        else
-          Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries (i,
-            i0) self.hashmap_hash_map_max_load self.hashmap_hash_map_slots)
-      end
-    end
-  end
+  let* max_usize = scalar_cast U32 Usize core_num_u32_max_c in
+  let capacity = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
+  let* n1 = usize_div max_usize 2 in
+  let (i, i0) = self.hashmap_hash_map_max_load_factor in
+  let* i1 = usize_div n1 i in
+  if capacity <= i1
+  then begin
+    let* i2 = usize_mul capacity 2 in
+    let* ntable = hashmap_hash_map_new_with_capacity_fwd t i2 i i0 in
+    let* (ntable0, _) =
+      hashmap_hash_map_move_elements_fwd_back t ntable
+        self.hashmap_hash_map_slots 0 in
+    Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries (i, i0)
+      ntable0.hashmap_hash_map_max_load ntable0.hashmap_hash_map_slots) end
+  else
+    Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries (i, i0)
+      self.hashmap_hash_map_max_load self.hashmap_hash_map_slots)
 
 (** [hashmap_main::hashmap::HashMap::{0}::insert] *)
 let hashmap_hash_map_insert_fwd_back
   (t : Type0) (self : hashmap_hash_map_t t) (key : usize) (value : t) :
   result (hashmap_hash_map_t t)
   =
-  begin match hashmap_hash_map_insert_no_resize_fwd_back t self key value with
-  | Fail e -> Fail e
-  | Return self0 ->
-    begin match hashmap_hash_map_len_fwd t self0 with
-    | Fail e -> Fail e
-    | Return i ->
-      if i > self0.hashmap_hash_map_max_load
-      then hashmap_hash_map_try_resize_fwd_back t self0
-      else Return self0
-    end
-  end
+  let* self0 = hashmap_hash_map_insert_no_resize_fwd_back t self key value in
+  let* i = hashmap_hash_map_len_fwd t self0 in
+  if i > self0.hashmap_hash_map_max_load
+  then hashmap_hash_map_try_resize_fwd_back t self0
+  else Return self0
 
 (** [hashmap_main::hashmap::HashMap::{0}::contains_key_in_list] *)
 let rec hashmap_hash_map_contains_key_in_list_loop_fwd
@@ -349,21 +256,12 @@ let hashmap_hash_map_contains_key_in_list_fwd
 (** [hashmap_main::hashmap::HashMap::{0}::contains_key] *)
 let hashmap_hash_map_contains_key_fwd
   (t : Type0) (self : hashmap_hash_map_t t) (key : usize) : result bool =
-  begin match hashmap_hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match
-        vec_index_fwd (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod
-        with
-      | Fail e -> Fail e
-      | Return l -> hashmap_hash_map_contains_key_in_list_fwd t key l
-      end
-    end
-  end
+  let* hash = hashmap_hash_key_fwd key in
+  let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l =
+    vec_index_fwd (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod in
+  hashmap_hash_map_contains_key_in_list_fwd t key l
 
 (** [hashmap_main::hashmap::HashMap::{0}::get_in_list] *)
 let rec hashmap_hash_map_get_in_list_loop_fwd
@@ -387,21 +285,12 @@ let hashmap_hash_map_get_in_list_fwd
 (** [hashmap_main::hashmap::HashMap::{0}::get] *)
 let hashmap_hash_map_get_fwd
   (t : Type0) (self : hashmap_hash_map_t t) (key : usize) : result t =
-  begin match hashmap_hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match
-        vec_index_fwd (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod
-        with
-      | Fail e -> Fail e
-      | Return l -> hashmap_hash_map_get_in_list_fwd t key l
-      end
-    end
-  end
+  let* hash = hashmap_hash_key_fwd key in
+  let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l =
+    vec_index_fwd (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod in
+  hashmap_hash_map_get_in_list_fwd t key l
 
 (** [hashmap_main::hashmap::HashMap::{0}::get_mut_in_list] *)
 let rec hashmap_hash_map_get_mut_in_list_loop_fwd
@@ -432,11 +321,9 @@ let rec hashmap_hash_map_get_mut_in_list_loop_back
   | HashmapListCons ckey cvalue tl ->
     if ckey = key
     then Return (HashmapListCons ckey ret tl)
-    else
-      begin match hashmap_hash_map_get_mut_in_list_loop_back t tl key ret with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (HashmapListCons ckey cvalue tl0)
-      end
+    else begin
+      let* tl0 = hashmap_hash_map_get_mut_in_list_loop_back t tl key ret in
+      Return (HashmapListCons ckey cvalue tl0) end
   | HashmapListNil -> Fail Failure
   end
 
@@ -450,55 +337,31 @@ let hashmap_hash_map_get_mut_in_list_back
 (** [hashmap_main::hashmap::HashMap::{0}::get_mut] *)
 let hashmap_hash_map_get_mut_fwd
   (t : Type0) (self : hashmap_hash_map_t t) (key : usize) : result t =
-  begin match hashmap_hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match
-        vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots
-          hash_mod with
-      | Fail e -> Fail e
-      | Return l -> hashmap_hash_map_get_mut_in_list_fwd t l key
-      end
-    end
-  end
+  let* hash = hashmap_hash_key_fwd key in
+  let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l =
+    vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod
+    in
+  hashmap_hash_map_get_mut_in_list_fwd t l key
 
 (** [hashmap_main::hashmap::HashMap::{0}::get_mut] *)
 let hashmap_hash_map_get_mut_back
   (t : Type0) (self : hashmap_hash_map_t t) (key : usize) (ret : t) :
   result (hashmap_hash_map_t t)
   =
-  begin match hashmap_hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match
-        vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots
-          hash_mod with
-      | Fail e -> Fail e
-      | Return l ->
-        begin match hashmap_hash_map_get_mut_in_list_back t l key ret with
-        | Fail e -> Fail e
-        | Return l0 ->
-          begin match
-            vec_index_mut_back (hashmap_list_t t) self.hashmap_hash_map_slots
-              hash_mod l0 with
-          | Fail e -> Fail e
-          | Return v ->
-            Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries
-              self.hashmap_hash_map_max_load_factor
-              self.hashmap_hash_map_max_load v)
-          end
-        end
-      end
-    end
-  end
+  let* hash = hashmap_hash_key_fwd key in
+  let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l =
+    vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod
+    in
+  let* l0 = hashmap_hash_map_get_mut_in_list_back t l key ret in
+  let* v =
+    vec_index_mut_back (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod
+      l0 in
+  Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries
+    self.hashmap_hash_map_max_load_factor self.hashmap_hash_map_max_load v)
 
 (** [hashmap_main::hashmap::HashMap::{0}::remove_from_list] *)
 let rec hashmap_hash_map_remove_from_list_loop_fwd
@@ -543,11 +406,9 @@ let rec hashmap_hash_map_remove_from_list_loop_back
       | HashmapListCons i cvalue tl0 -> Return tl0
       | HashmapListNil -> Fail Failure
       end
-    else
-      begin match hashmap_hash_map_remove_from_list_loop_back t key tl with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (HashmapListCons ckey x tl0)
-      end
+    else begin
+      let* tl0 = hashmap_hash_map_remove_from_list_loop_back t key tl in
+      Return (HashmapListCons ckey x tl0) end
   | HashmapListNil -> Return HashmapListNil
   end
 
@@ -561,32 +422,17 @@ let hashmap_hash_map_remove_from_list_back
 (** [hashmap_main::hashmap::HashMap::{0}::remove] *)
 let hashmap_hash_map_remove_fwd
   (t : Type0) (self : hashmap_hash_map_t t) (key : usize) : result (option t) =
-  begin match hashmap_hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match
-        vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots
-          hash_mod with
-      | Fail e -> Fail e
-      | Return l ->
-        begin match hashmap_hash_map_remove_from_list_fwd t key l with
-        | Fail e -> Fail e
-        | Return x ->
-          begin match x with
-          | None -> Return None
-          | Some x0 ->
-            begin match usize_sub self.hashmap_hash_map_num_entries 1 with
-            | Fail e -> Fail e
-            | Return _ -> Return (Some x0)
-            end
-          end
-        end
-      end
-    end
+  let* hash = hashmap_hash_key_fwd key in
+  let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l =
+    vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod
+    in
+  let* x = hashmap_hash_map_remove_from_list_fwd t key l in
+  begin match x with
+  | None -> Return None
+  | Some x0 ->
+    let* _ = usize_sub self.hashmap_hash_map_num_entries 1 in Return (Some x0)
   end
 
 (** [hashmap_main::hashmap::HashMap::{0}::remove] *)
@@ -594,141 +440,66 @@ let hashmap_hash_map_remove_back
   (t : Type0) (self : hashmap_hash_map_t t) (key : usize) :
   result (hashmap_hash_map_t t)
   =
-  begin match hashmap_hash_key_fwd key with
-  | Fail e -> Fail e
-  | Return hash ->
-    let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
-    begin match usize_rem hash i with
-    | Fail e -> Fail e
-    | Return hash_mod ->
-      begin match
-        vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots
-          hash_mod with
-      | Fail e -> Fail e
-      | Return l ->
-        begin match hashmap_hash_map_remove_from_list_fwd t key l with
-        | Fail e -> Fail e
-        | Return x ->
-          begin match x with
-          | None ->
-            begin match hashmap_hash_map_remove_from_list_back t key l with
-            | Fail e -> Fail e
-            | Return l0 ->
-              begin match
-                vec_index_mut_back (hashmap_list_t t)
-                  self.hashmap_hash_map_slots hash_mod l0 with
-              | Fail e -> Fail e
-              | Return v ->
-                Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries
-                  self.hashmap_hash_map_max_load_factor
-                  self.hashmap_hash_map_max_load v)
-              end
-            end
-          | Some x0 ->
-            begin match usize_sub self.hashmap_hash_map_num_entries 1 with
-            | Fail e -> Fail e
-            | Return i0 ->
-              begin match hashmap_hash_map_remove_from_list_back t key l with
-              | Fail e -> Fail e
-              | Return l0 ->
-                begin match
-                  vec_index_mut_back (hashmap_list_t t)
-                    self.hashmap_hash_map_slots hash_mod l0 with
-                | Fail e -> Fail e
-                | Return v ->
-                  Return (Mkhashmap_hash_map_t i0
-                    self.hashmap_hash_map_max_load_factor
-                    self.hashmap_hash_map_max_load v)
-                end
-              end
-            end
-          end
-        end
-      end
-    end
+  let* hash = hashmap_hash_key_fwd key in
+  let i = vec_len (hashmap_list_t t) self.hashmap_hash_map_slots in
+  let* hash_mod = usize_rem hash i in
+  let* l =
+    vec_index_mut_fwd (hashmap_list_t t) self.hashmap_hash_map_slots hash_mod
+    in
+  let* x = hashmap_hash_map_remove_from_list_fwd t key l in
+  begin match x with
+  | None ->
+    let* l0 = hashmap_hash_map_remove_from_list_back t key l in
+    let* v =
+      vec_index_mut_back (hashmap_list_t t) self.hashmap_hash_map_slots
+        hash_mod l0 in
+    Return (Mkhashmap_hash_map_t self.hashmap_hash_map_num_entries
+      self.hashmap_hash_map_max_load_factor self.hashmap_hash_map_max_load v)
+  | Some x0 ->
+    let* i0 = usize_sub self.hashmap_hash_map_num_entries 1 in
+    let* l0 = hashmap_hash_map_remove_from_list_back t key l in
+    let* v =
+      vec_index_mut_back (hashmap_list_t t) self.hashmap_hash_map_slots
+        hash_mod l0 in
+    Return (Mkhashmap_hash_map_t i0 self.hashmap_hash_map_max_load_factor
+      self.hashmap_hash_map_max_load v)
   end
 
 (** [hashmap_main::hashmap::test1] *)
 let hashmap_test1_fwd : result unit =
-  begin match hashmap_hash_map_new_fwd u64 with
-  | Fail e -> Fail e
-  | Return hm ->
-    begin match hashmap_hash_map_insert_fwd_back u64 hm 0 42 with
-    | Fail e -> Fail e
-    | Return hm0 ->
-      begin match hashmap_hash_map_insert_fwd_back u64 hm0 128 18 with
-      | Fail e -> Fail e
-      | Return hm1 ->
-        begin match hashmap_hash_map_insert_fwd_back u64 hm1 1024 138 with
-        | Fail e -> Fail e
-        | Return hm2 ->
-          begin match hashmap_hash_map_insert_fwd_back u64 hm2 1056 256 with
-          | Fail e -> Fail e
-          | Return hm3 ->
-            begin match hashmap_hash_map_get_fwd u64 hm3 128 with
-            | Fail e -> Fail e
-            | Return i ->
-              if not (i = 18)
-              then Fail Failure
-              else
-                begin match hashmap_hash_map_get_mut_back u64 hm3 1024 56 with
-                | Fail e -> Fail e
-                | Return hm4 ->
-                  begin match hashmap_hash_map_get_fwd u64 hm4 1024 with
-                  | Fail e -> Fail e
-                  | Return i0 ->
-                    if not (i0 = 56)
-                    then Fail Failure
-                    else
-                      begin match hashmap_hash_map_remove_fwd u64 hm4 1024 with
-                      | Fail e -> Fail e
-                      | Return x ->
-                        begin match x with
-                        | None -> Fail Failure
-                        | Some x0 ->
-                          if not (x0 = 56)
-                          then Fail Failure
-                          else
-                            begin match
-                              hashmap_hash_map_remove_back u64 hm4 1024 with
-                            | Fail e -> Fail e
-                            | Return hm5 ->
-                              begin match hashmap_hash_map_get_fwd u64 hm5 0
-                                with
-                              | Fail e -> Fail e
-                              | Return i1 ->
-                                if not (i1 = 42)
-                                then Fail Failure
-                                else
-                                  begin match
-                                    hashmap_hash_map_get_fwd u64 hm5 128 with
-                                  | Fail e -> Fail e
-                                  | Return i2 ->
-                                    if not (i2 = 18)
-                                    then Fail Failure
-                                    else
-                                      begin match
-                                        hashmap_hash_map_get_fwd u64 hm5 1056
-                                        with
-                                      | Fail e -> Fail e
-                                      | Return i3 ->
-                                        if not (i3 = 256)
-                                        then Fail Failure
-                                        else Return ()
-                                      end
-                                  end
-                              end
-                            end
-                        end
-                      end
-                  end
-                end
-            end
-          end
-        end
-      end
-    end
-  end
+  let* hm = hashmap_hash_map_new_fwd u64 in
+  let* hm0 = hashmap_hash_map_insert_fwd_back u64 hm 0 42 in
+  let* hm1 = hashmap_hash_map_insert_fwd_back u64 hm0 128 18 in
+  let* hm2 = hashmap_hash_map_insert_fwd_back u64 hm1 1024 138 in
+  let* hm3 = hashmap_hash_map_insert_fwd_back u64 hm2 1056 256 in
+  let* i = hashmap_hash_map_get_fwd u64 hm3 128 in
+  if not (i = 18)
+  then Fail Failure
+  else begin
+    let* hm4 = hashmap_hash_map_get_mut_back u64 hm3 1024 56 in
+    let* i0 = hashmap_hash_map_get_fwd u64 hm4 1024 in
+    if not (i0 = 56)
+    then Fail Failure
+    else begin
+      let* x = hashmap_hash_map_remove_fwd u64 hm4 1024 in
+      begin match x with
+      | None -> Fail Failure
+      | Some x0 ->
+        if not (x0 = 56)
+        then Fail Failure
+        else begin
+          let* hm5 = hashmap_hash_map_remove_back u64 hm4 1024 in
+          let* i1 = hashmap_hash_map_get_fwd u64 hm5 0 in
+          if not (i1 = 42)
+          then Fail Failure
+          else begin
+            let* i2 = hashmap_hash_map_get_fwd u64 hm5 128 in
+            if not (i2 = 18)
+            then Fail Failure
+            else begin
+              let* i3 = hashmap_hash_map_get_fwd u64 hm5 1056 in
+              if not (i3 = 256) then Fail Failure else Return () end end end
+      end end end
 
 (** Unit test for [hashmap_main::hashmap::test1] *)
 let _ = assert_norm (hashmap_test1_fwd = Return ())
@@ -736,18 +507,10 @@ let _ = assert_norm (hashmap_test1_fwd = Return ())
 (** [hashmap_main::insert_on_disk] *)
 let insert_on_disk_fwd
   (key : usize) (value : u64) (st : state) : result (state & unit) =
-  begin match hashmap_utils_deserialize_fwd st with
-  | Fail e -> Fail e
-  | Return (st0, hm) ->
-    begin match hashmap_hash_map_insert_fwd_back u64 hm key value with
-    | Fail e -> Fail e
-    | Return hm0 ->
-      begin match hashmap_utils_serialize_fwd hm0 st0 with
-      | Fail e -> Fail e
-      | Return (st1, _) -> Return (st1, ())
-      end
-    end
-  end
+  let* (st0, hm) = hashmap_utils_deserialize_fwd st in
+  let* hm0 = hashmap_hash_map_insert_fwd_back u64 hm key value in
+  let* (st1, _) = hashmap_utils_serialize_fwd hm0 st0 in
+  Return (st1, ())
 
 (** [hashmap_main::main] *)
 let main_fwd : result unit = Return ()
diff --git a/tests/fstar/hashmap_on_disk/Primitives.fst b/tests/fstar/hashmap_on_disk/Primitives.fst
index bf0f9078..98a696b6 100644
--- a/tests/fstar/hashmap_on_disk/Primitives.fst
+++ b/tests/fstar/hashmap_on_disk/Primitives.fst
@@ -35,7 +35,9 @@ unfold let return (#a : Type0) (x : a) : result a = Return x
 // let* x = y in
 // ...
 // ```
-unfold let (let*) (#a #b : Type0) (m: result a) (f: a -> result b) : result b =
+unfold let (let*) (#a #b : Type0) (m: result a)
+  (f: (x:a) -> Pure (result b) (requires (m == Return x)) (ensures fun _ -> True)) :
+  result b =
   match m with
   | Return x -> f x
   | Fail e   -> Fail e
diff --git a/tests/fstar/misc/Constants.fst b/tests/fstar/misc/Constants.fst
index 9b90e9c7..1a2f4133 100644
--- a/tests/fstar/misc/Constants.fst
+++ b/tests/fstar/misc/Constants.fst
@@ -96,14 +96,7 @@ let q3_c : i32 = eval_global q3_body
 
 (** [constants::get_z2] *)
 let get_z2_fwd : result i32 =
-  begin match get_z1_fwd with
-  | Fail e -> Fail e
-  | Return i ->
-    begin match add_fwd i q3_c with
-    | Fail e -> Fail e
-    | Return i0 -> add_fwd q1_c i0
-    end
-  end
+  let* i = get_z1_fwd in let* i0 = add_fwd i q3_c in add_fwd q1_c i0
 
 (** [constants::S1] *)
 let s1_body : result u32 = Return 6
diff --git a/tests/fstar/misc/External.Funs.fst b/tests/fstar/misc/External.Funs.fst
index 2529ec33..f70a9fc6 100644
--- a/tests/fstar/misc/External.Funs.fst
+++ b/tests/fstar/misc/External.Funs.fst
@@ -9,53 +9,30 @@ include External.Opaque
 
 (** [external::swap] *)
 let swap_fwd (t : Type0) (x : t) (y : t) (st : state) : result (state & unit) =
-  begin match core_mem_swap_fwd t x y st with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match core_mem_swap_back0 t x y st st0 with
-    | Fail e -> Fail e
-    | Return (st1, _) ->
-      begin match core_mem_swap_back1 t x y st st1 with
-      | Fail e -> Fail e
-      | Return (st2, _) -> Return (st2, ())
-      end
-    end
-  end
+  let* (st0, _) = core_mem_swap_fwd t x y st in
+  let* (st1, _) = core_mem_swap_back0 t x y st st0 in
+  let* (st2, _) = core_mem_swap_back1 t x y st st1 in
+  Return (st2, ())
 
 (** [external::swap] *)
 let swap_back
   (t : Type0) (x : t) (y : t) (st : state) (st0 : state) :
   result (state & (t & t))
   =
-  begin match core_mem_swap_fwd t x y st with
-  | Fail e -> Fail e
-  | Return (st1, _) ->
-    begin match core_mem_swap_back0 t x y st st1 with
-    | Fail e -> Fail e
-    | Return (st2, x0) ->
-      begin match core_mem_swap_back1 t x y st st2 with
-      | Fail e -> Fail e
-      | Return (_, y0) -> Return (st0, (x0, y0))
-      end
-    end
-  end
+  let* (st1, _) = core_mem_swap_fwd t x y st in
+  let* (st2, x0) = core_mem_swap_back0 t x y st st1 in
+  let* (_, y0) = core_mem_swap_back1 t x y st st2 in
+  Return (st0, (x0, y0))
 
 (** [external::test_new_non_zero_u32] *)
 let test_new_non_zero_u32_fwd
   (x : u32) (st : state) : result (state & core_num_nonzero_non_zero_u32_t) =
-  begin match core_num_nonzero_non_zero_u32_new_fwd x st with
-  | Fail e -> Fail e
-  | Return (st0, opt) ->
-    core_option_option_unwrap_fwd core_num_nonzero_non_zero_u32_t opt st0
-  end
+  let* (st0, opt) = core_num_nonzero_non_zero_u32_new_fwd x st in
+  core_option_option_unwrap_fwd core_num_nonzero_non_zero_u32_t opt st0
 
 (** [external::test_vec] *)
 let test_vec_fwd : result unit =
-  let v = vec_new u32 in
-  begin match vec_push_back u32 v 0 with
-  | Fail e -> Fail e
-  | Return _ -> Return ()
-  end
+  let v = vec_new u32 in let* _ = vec_push_back u32 v 0 in Return ()
 
 (** Unit test for [external::test_vec] *)
 let _ = assert_norm (test_vec_fwd = Return ())
@@ -63,44 +40,25 @@ let _ = assert_norm (test_vec_fwd = Return ())
 (** [external::custom_swap] *)
 let custom_swap_fwd
   (t : Type0) (x : t) (y : t) (st : state) : result (state & t) =
-  begin match core_mem_swap_fwd t x y st with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match core_mem_swap_back0 t x y st st0 with
-    | Fail e -> Fail e
-    | Return (st1, x0) ->
-      begin match core_mem_swap_back1 t x y st st1 with
-      | Fail e -> Fail e
-      | Return (st2, _) -> Return (st2, x0)
-      end
-    end
-  end
+  let* (st0, _) = core_mem_swap_fwd t x y st in
+  let* (st1, x0) = core_mem_swap_back0 t x y st st0 in
+  let* (st2, _) = core_mem_swap_back1 t x y st st1 in
+  Return (st2, x0)
 
 (** [external::custom_swap] *)
 let custom_swap_back
   (t : Type0) (x : t) (y : t) (st : state) (ret : t) (st0 : state) :
   result (state & (t & t))
   =
-  begin match core_mem_swap_fwd t x y st with
-  | Fail e -> Fail e
-  | Return (st1, _) ->
-    begin match core_mem_swap_back0 t x y st st1 with
-    | Fail e -> Fail e
-    | Return (st2, _) ->
-      begin match core_mem_swap_back1 t x y st st2 with
-      | Fail e -> Fail e
-      | Return (_, y0) -> Return (st0, (ret, y0))
-      end
-    end
-  end
+  let* (st1, _) = core_mem_swap_fwd t x y st in
+  let* (st2, _) = core_mem_swap_back0 t x y st st1 in
+  let* (_, y0) = core_mem_swap_back1 t x y st st2 in
+  Return (st0, (ret, y0))
 
 (** [external::test_custom_swap] *)
 let test_custom_swap_fwd
   (x : u32) (y : u32) (st : state) : result (state & unit) =
-  begin match custom_swap_fwd u32 x y st with
-  | Fail e -> Fail e
-  | Return (st0, _) -> Return (st0, ())
-  end
+  let* (st0, _) = custom_swap_fwd u32 x y st in Return (st0, ())
 
 (** [external::test_custom_swap] *)
 let test_custom_swap_back
@@ -111,13 +69,7 @@ let test_custom_swap_back
 
 (** [external::test_swap_non_zero] *)
 let test_swap_non_zero_fwd (x : u32) (st : state) : result (state & u32) =
-  begin match swap_fwd u32 x 0 st with
-  | Fail e -> Fail e
-  | Return (st0, _) ->
-    begin match swap_back u32 x 0 st st0 with
-    | Fail e -> Fail e
-    | Return (st1, (x0, _)) ->
-      if x0 = 0 then Fail Failure else Return (st1, x0)
-    end
-  end
+  let* (st0, _) = swap_fwd u32 x 0 st in
+  let* (st1, (x0, _)) = swap_back u32 x 0 st st0 in
+  if x0 = 0 then Fail Failure else Return (st1, x0)
 
diff --git a/tests/fstar/misc/Loops.Funs.fst b/tests/fstar/misc/Loops.Funs.fst
index 3e8425dd..0d3c39f7 100644
--- a/tests/fstar/misc/Loops.Funs.fst
+++ b/tests/fstar/misc/Loops.Funs.fst
@@ -13,14 +13,8 @@ let rec sum_loop_fwd
   Tot (result u32) (decreases (sum_loop_decreases max i s))
   =
   if i < max
-  then
-    begin match u32_add s i with
-    | Fail e -> Fail e
-    | Return s0 ->
-      begin match u32_add i 1 with
-      | Fail e -> Fail e
-      | Return i0 -> sum_loop_fwd max i0 s0
-      end
+  then begin
+    let* s0 = u32_add s i in let* i0 = u32_add i 1 in sum_loop_fwd max i0 s0
     end
   else u32_mul s 2
 
@@ -33,15 +27,10 @@ let rec sum_with_mut_borrows_loop_fwd
   Tot (result u32) (decreases (sum_with_mut_borrows_loop_decreases max mi ms))
   =
   if mi < max
-  then
-    begin match u32_add ms mi with
-    | Fail e -> Fail e
-    | Return ms0 ->
-      begin match u32_add mi 1 with
-      | Fail e -> Fail e
-      | Return mi0 -> sum_with_mut_borrows_loop_fwd max mi0 ms0
-      end
-    end
+  then begin
+    let* ms0 = u32_add ms mi in
+    let* mi0 = u32_add mi 1 in
+    sum_with_mut_borrows_loop_fwd max mi0 ms0 end
   else u32_mul ms 2
 
 (** [loops::sum_with_mut_borrows] *)
@@ -54,15 +43,10 @@ let rec sum_with_shared_borrows_loop_fwd
   Tot (result u32) (decreases (sum_with_shared_borrows_loop_decreases max i s))
   =
   if i < max
-  then
-    begin match u32_add i 1 with
-    | Fail e -> Fail e
-    | Return i0 ->
-      begin match u32_add s i0 with
-      | Fail e -> Fail e
-      | Return s0 -> sum_with_shared_borrows_loop_fwd max i0 s0
-      end
-    end
+  then begin
+    let* i0 = u32_add i 1 in
+    let* s0 = u32_add s i0 in
+    sum_with_shared_borrows_loop_fwd max i0 s0 end
   else u32_mul s 2
 
 (** [loops::sum_with_shared_borrows] *)
@@ -76,15 +60,10 @@ let rec clear_loop_fwd_back
   =
   let i0 = vec_len u32 v in
   if i < i0
-  then
-    begin match usize_add i 1 with
-    | Fail e -> Fail e
-    | Return i1 ->
-      begin match vec_index_mut_back u32 v i 0 with
-      | Fail e -> Fail e
-      | Return v0 -> clear_loop_fwd_back v0 i1
-      end
-    end
+  then begin
+    let* i1 = usize_add i 1 in
+    let* v0 = vec_index_mut_back u32 v i 0 in
+    clear_loop_fwd_back v0 i1 end
   else Return v
 
 (** [loops::clear] *)
@@ -113,11 +92,7 @@ let rec list_nth_mut_loop_loop_fwd
   | ListCons x tl ->
     if i = 0
     then Return x
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 -> list_nth_mut_loop_loop_fwd t tl i0
-      end
+    else begin let* i0 = u32_sub i 1 in list_nth_mut_loop_loop_fwd t tl i0 end
   | ListNil -> Fail Failure
   end
 
@@ -134,15 +109,10 @@ let rec list_nth_mut_loop_loop_back
   | ListCons x tl ->
     if i = 0
     then Return (ListCons ret tl)
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 ->
-        begin match list_nth_mut_loop_loop_back t tl i0 ret with
-        | Fail e -> Fail e
-        | Return tl0 -> Return (ListCons x tl0)
-        end
-      end
+    else begin
+      let* i0 = u32_sub i 1 in
+      let* tl0 = list_nth_mut_loop_loop_back t tl i0 ret in
+      Return (ListCons x tl0) end
   | ListNil -> Fail Failure
   end
 
@@ -160,10 +130,7 @@ let rec list_nth_shared_loop_loop_fwd
   | ListCons x tl ->
     if i = 0
     then Return x
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 -> list_nth_shared_loop_loop_fwd t tl i0
+    else begin let* i0 = u32_sub i 1 in list_nth_shared_loop_loop_fwd t tl i0
       end
   | ListNil -> Fail Failure
   end
@@ -184,10 +151,8 @@ let rec get_elem_mut_loop_fwd
 
 (** [loops::get_elem_mut] *)
 let get_elem_mut_fwd (slots : vec (list_t usize)) (x : usize) : result usize =
-  begin match vec_index_mut_fwd (list_t usize) slots 0 with
-  | Fail e -> Fail e
-  | Return l -> get_elem_mut_loop_fwd x l
-  end
+  let* l = vec_index_mut_fwd (list_t usize) slots 0 in
+  get_elem_mut_loop_fwd x l
 
 (** [loops::get_elem_mut] *)
 let rec get_elem_mut_loop_back
@@ -198,11 +163,8 @@ let rec get_elem_mut_loop_back
   | ListCons y tl ->
     if y = x
     then Return (ListCons ret tl)
-    else
-      begin match get_elem_mut_loop_back x tl ret with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (ListCons y tl0)
-      end
+    else begin
+      let* tl0 = get_elem_mut_loop_back x tl ret in Return (ListCons y tl0) end
   | ListNil -> Fail Failure
   end
 
@@ -211,14 +173,9 @@ let get_elem_mut_back
   (slots : vec (list_t usize)) (x : usize) (ret : usize) :
   result (vec (list_t usize))
   =
-  begin match vec_index_mut_fwd (list_t usize) slots 0 with
-  | Fail e -> Fail e
-  | Return l ->
-    begin match get_elem_mut_loop_back x l ret with
-    | Fail e -> Fail e
-    | Return l0 -> vec_index_mut_back (list_t usize) slots 0 l0
-    end
-  end
+  let* l = vec_index_mut_fwd (list_t usize) slots 0 in
+  let* l0 = get_elem_mut_loop_back x l ret in
+  vec_index_mut_back (list_t usize) slots 0 l0
 
 (** [loops::get_elem_shared] *)
 let rec get_elem_shared_loop_fwd
@@ -233,10 +190,7 @@ let rec get_elem_shared_loop_fwd
 (** [loops::get_elem_shared] *)
 let get_elem_shared_fwd
   (slots : vec (list_t usize)) (x : usize) : result usize =
-  begin match vec_index_fwd (list_t usize) slots 0 with
-  | Fail e -> Fail e
-  | Return l -> get_elem_shared_loop_fwd x l
-  end
+  let* l = vec_index_fwd (list_t usize) slots 0 in get_elem_shared_loop_fwd x l
 
 (** [loops::id_mut] *)
 let id_mut_fwd (t : Type0) (ls : list_t t) : result (list_t t) = Return ls
@@ -258,21 +212,15 @@ let rec list_nth_mut_loop_with_id_loop_fwd
   | ListCons x tl ->
     if i = 0
     then Return x
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 -> list_nth_mut_loop_with_id_loop_fwd t i0 tl
-      end
+    else begin
+      let* i0 = u32_sub i 1 in list_nth_mut_loop_with_id_loop_fwd t i0 tl end
   | ListNil -> Fail Failure
   end
 
 (** [loops::list_nth_mut_loop_with_id] *)
 let list_nth_mut_loop_with_id_fwd
   (t : Type0) (ls : list_t t) (i : u32) : result t =
-  begin match id_mut_fwd t ls with
-  | Fail e -> Fail e
-  | Return ls0 -> list_nth_mut_loop_with_id_loop_fwd t i ls0
-  end
+  let* ls0 = id_mut_fwd t ls in list_nth_mut_loop_with_id_loop_fwd t i ls0
 
 (** [loops::list_nth_mut_loop_with_id] *)
 let rec list_nth_mut_loop_with_id_loop_back
@@ -284,29 +232,19 @@ let rec list_nth_mut_loop_with_id_loop_back
   | ListCons x tl ->
     if i = 0
     then Return (ListCons ret tl)
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 ->
-        begin match list_nth_mut_loop_with_id_loop_back t i0 tl ret with
-        | Fail e -> Fail e
-        | Return tl0 -> Return (ListCons x tl0)
-        end
-      end
+    else begin
+      let* i0 = u32_sub i 1 in
+      let* tl0 = list_nth_mut_loop_with_id_loop_back t i0 tl ret in
+      Return (ListCons x tl0) end
   | ListNil -> Fail Failure
   end
 
 (** [loops::list_nth_mut_loop_with_id] *)
 let list_nth_mut_loop_with_id_back
   (t : Type0) (ls : list_t t) (i : u32) (ret : t) : result (list_t t) =
-  begin match id_mut_fwd t ls with
-  | Fail e -> Fail e
-  | Return ls0 ->
-    begin match list_nth_mut_loop_with_id_loop_back t i ls0 ret with
-    | Fail e -> Fail e
-    | Return l -> id_mut_back t ls l
-    end
-  end
+  let* ls0 = id_mut_fwd t ls in
+  let* l = list_nth_mut_loop_with_id_loop_back t i ls0 ret in
+  id_mut_back t ls l
 
 (** [loops::list_nth_shared_loop_with_id] *)
 let rec list_nth_shared_loop_with_id_loop_fwd
@@ -318,10 +256,8 @@ let rec list_nth_shared_loop_with_id_loop_fwd
   | ListCons x tl ->
     if i = 0
     then Return x
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 -> list_nth_shared_loop_with_id_loop_fwd t i0 tl
+    else begin
+      let* i0 = u32_sub i 1 in list_nth_shared_loop_with_id_loop_fwd t i0 tl
       end
   | ListNil -> Fail Failure
   end
@@ -329,10 +265,8 @@ let rec list_nth_shared_loop_with_id_loop_fwd
 (** [loops::list_nth_shared_loop_with_id] *)
 let list_nth_shared_loop_with_id_fwd
   (t : Type0) (ls : list_t t) (i : u32) : result t =
-  begin match id_shared_fwd t ls with
-  | Fail e -> Fail e
-  | Return ls0 -> list_nth_shared_loop_with_id_loop_fwd t i ls0
-  end
+  let* ls0 = id_shared_fwd t ls in
+  list_nth_shared_loop_with_id_loop_fwd t i ls0
 
 (** [loops::list_nth_mut_loop_pair] *)
 let rec list_nth_mut_loop_pair_loop_fwd
@@ -346,10 +280,8 @@ let rec list_nth_mut_loop_pair_loop_fwd
     | ListCons x1 tl1 ->
       if i = 0
       then Return (x0, x1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 -> list_nth_mut_loop_pair_loop_fwd t tl0 tl1 i0
+      else begin
+        let* i0 = u32_sub i 1 in list_nth_mut_loop_pair_loop_fwd t tl0 tl1 i0
         end
     | ListNil -> Fail Failure
     end
@@ -373,15 +305,10 @@ let rec list_nth_mut_loop_pair_loop_back'a
     | ListCons x1 tl1 ->
       if i = 0
       then Return (ListCons ret tl0)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 ->
-          begin match list_nth_mut_loop_pair_loop_back'a t tl0 tl1 i0 ret with
-          | Fail e -> Fail e
-          | Return tl00 -> Return (ListCons x0 tl00)
-          end
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        let* tl00 = list_nth_mut_loop_pair_loop_back'a t tl0 tl1 i0 ret in
+        Return (ListCons x0 tl00) end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -406,15 +333,10 @@ let rec list_nth_mut_loop_pair_loop_back'b
     | ListCons x1 tl1 ->
       if i = 0
       then Return (ListCons ret tl1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 ->
-          begin match list_nth_mut_loop_pair_loop_back'b t tl0 tl1 i0 ret with
-          | Fail e -> Fail e
-          | Return tl10 -> Return (ListCons x1 tl10)
-          end
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        let* tl10 = list_nth_mut_loop_pair_loop_back'b t tl0 tl1 i0 ret in
+        Return (ListCons x1 tl10) end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -439,11 +361,9 @@ let rec list_nth_shared_loop_pair_loop_fwd
     | ListCons x1 tl1 ->
       if i = 0
       then Return (x0, x1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 -> list_nth_shared_loop_pair_loop_fwd t tl0 tl1 i0
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        list_nth_shared_loop_pair_loop_fwd t tl0 tl1 i0 end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -466,11 +386,9 @@ let rec list_nth_mut_loop_pair_merge_loop_fwd
     | ListCons x1 tl1 ->
       if i = 0
       then Return (x0, x1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 -> list_nth_mut_loop_pair_merge_loop_fwd t tl0 tl1 i0
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        list_nth_mut_loop_pair_merge_loop_fwd t tl0 tl1 i0 end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -493,16 +411,11 @@ let rec list_nth_mut_loop_pair_merge_loop_back
     | ListCons x1 tl1 ->
       if i = 0
       then let (x, x2) = ret in Return (ListCons x tl0, ListCons x2 tl1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 ->
-          begin match list_nth_mut_loop_pair_merge_loop_back t tl0 tl1 i0 ret
-            with
-          | Fail e -> Fail e
-          | Return (tl00, tl10) -> Return (ListCons x0 tl00, ListCons x1 tl10)
-          end
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        let* (tl00, tl10) =
+          list_nth_mut_loop_pair_merge_loop_back t tl0 tl1 i0 ret in
+        Return (ListCons x0 tl00, ListCons x1 tl10) end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -527,11 +440,9 @@ let rec list_nth_shared_loop_pair_merge_loop_fwd
     | ListCons x1 tl1 ->
       if i = 0
       then Return (x0, x1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 -> list_nth_shared_loop_pair_merge_loop_fwd t tl0 tl1 i0
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        list_nth_shared_loop_pair_merge_loop_fwd t tl0 tl1 i0 end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -554,11 +465,9 @@ let rec list_nth_mut_shared_loop_pair_loop_fwd
     | ListCons x1 tl1 ->
       if i = 0
       then Return (x0, x1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 -> list_nth_mut_shared_loop_pair_loop_fwd t tl0 tl1 i0
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        list_nth_mut_shared_loop_pair_loop_fwd t tl0 tl1 i0 end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -581,16 +490,10 @@ let rec list_nth_mut_shared_loop_pair_loop_back
     | ListCons x1 tl1 ->
       if i = 0
       then Return (ListCons ret tl0)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 ->
-          begin match list_nth_mut_shared_loop_pair_loop_back t tl0 tl1 i0 ret
-            with
-          | Fail e -> Fail e
-          | Return tl00 -> Return (ListCons x0 tl00)
-          end
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        let* tl00 = list_nth_mut_shared_loop_pair_loop_back t tl0 tl1 i0 ret in
+        Return (ListCons x0 tl00) end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -615,12 +518,9 @@ let rec list_nth_mut_shared_loop_pair_merge_loop_fwd
     | ListCons x1 tl1 ->
       if i = 0
       then Return (x0, x1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 ->
-          list_nth_mut_shared_loop_pair_merge_loop_fwd t tl0 tl1 i0
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        list_nth_mut_shared_loop_pair_merge_loop_fwd t tl0 tl1 i0 end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -643,16 +543,11 @@ let rec list_nth_mut_shared_loop_pair_merge_loop_back
     | ListCons x1 tl1 ->
       if i = 0
       then Return (ListCons ret tl0)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 ->
-          begin match
-            list_nth_mut_shared_loop_pair_merge_loop_back t tl0 tl1 i0 ret with
-          | Fail e -> Fail e
-          | Return tl00 -> Return (ListCons x0 tl00)
-          end
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        let* tl00 =
+          list_nth_mut_shared_loop_pair_merge_loop_back t tl0 tl1 i0 ret in
+        Return (ListCons x0 tl00) end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -677,11 +572,9 @@ let rec list_nth_shared_mut_loop_pair_loop_fwd
     | ListCons x1 tl1 ->
       if i = 0
       then Return (x0, x1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 -> list_nth_shared_mut_loop_pair_loop_fwd t tl0 tl1 i0
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        list_nth_shared_mut_loop_pair_loop_fwd t tl0 tl1 i0 end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -704,16 +597,10 @@ let rec list_nth_shared_mut_loop_pair_loop_back
     | ListCons x1 tl1 ->
       if i = 0
       then Return (ListCons ret tl1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 ->
-          begin match list_nth_shared_mut_loop_pair_loop_back t tl0 tl1 i0 ret
-            with
-          | Fail e -> Fail e
-          | Return tl10 -> Return (ListCons x1 tl10)
-          end
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        let* tl10 = list_nth_shared_mut_loop_pair_loop_back t tl0 tl1 i0 ret in
+        Return (ListCons x1 tl10) end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -738,12 +625,9 @@ let rec list_nth_shared_mut_loop_pair_merge_loop_fwd
     | ListCons x1 tl1 ->
       if i = 0
       then Return (x0, x1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 ->
-          list_nth_shared_mut_loop_pair_merge_loop_fwd t tl0 tl1 i0
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        list_nth_shared_mut_loop_pair_merge_loop_fwd t tl0 tl1 i0 end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
@@ -766,16 +650,11 @@ let rec list_nth_shared_mut_loop_pair_merge_loop_back
     | ListCons x1 tl1 ->
       if i = 0
       then Return (ListCons ret tl1)
-      else
-        begin match u32_sub i 1 with
-        | Fail e -> Fail e
-        | Return i0 ->
-          begin match
-            list_nth_shared_mut_loop_pair_merge_loop_back t tl0 tl1 i0 ret with
-          | Fail e -> Fail e
-          | Return tl10 -> Return (ListCons x1 tl10)
-          end
-        end
+      else begin
+        let* i0 = u32_sub i 1 in
+        let* tl10 =
+          list_nth_shared_mut_loop_pair_merge_loop_back t tl0 tl1 i0 ret in
+        Return (ListCons x1 tl10) end
     | ListNil -> Fail Failure
     end
   | ListNil -> Fail Failure
diff --git a/tests/fstar/misc/NoNestedBorrows.fst b/tests/fstar/misc/NoNestedBorrows.fst
index f8c63798..ce1f544c 100644
--- a/tests/fstar/misc/NoNestedBorrows.fst
+++ b/tests/fstar/misc/NoNestedBorrows.fst
@@ -52,8 +52,7 @@ let rem_test_fwd (x : u32) (y : u32) : result u32 = u32_rem x y
 let cast_test_fwd (x : u32) : result i32 = scalar_cast U32 I32 x
 
 (** [no_nested_borrows::test2] *)
-let test2_fwd : result unit =
-  begin match u32_add 23 44 with | Fail e -> Fail e | Return _ -> Return () end
+let test2_fwd : result unit = let* _ = u32_add 23 44 in Return ()
 
 (** Unit test for [no_nested_borrows::test2] *)
 let _ = assert_norm (test2_fwd = Return ())
@@ -64,28 +63,17 @@ let get_max_fwd (x : u32) (y : u32) : result u32 =
 
 (** [no_nested_borrows::test3] *)
 let test3_fwd : result unit =
-  begin match get_max_fwd 4 3 with
-  | Fail e -> Fail e
-  | Return x ->
-    begin match get_max_fwd 10 11 with
-    | Fail e -> Fail e
-    | Return y ->
-      begin match u32_add x y with
-      | Fail e -> Fail e
-      | Return z -> if not (z = 15) then Fail Failure else Return ()
-      end
-    end
-  end
+  let* x = get_max_fwd 4 3 in
+  let* y = get_max_fwd 10 11 in
+  let* z = u32_add x y in
+  if not (z = 15) then Fail Failure else Return ()
 
 (** Unit test for [no_nested_borrows::test3] *)
 let _ = assert_norm (test3_fwd = Return ())
 
 (** [no_nested_borrows::test_neg1] *)
 let test_neg1_fwd : result unit =
-  begin match i32_neg 3 with
-  | Fail e -> Fail e
-  | Return y -> if not (y = -3) then Fail Failure else Return ()
-  end
+  let* y = i32_neg 3 in if not (y = -3) then Fail Failure else Return ()
 
 (** Unit test for [no_nested_borrows::test_neg1] *)
 let _ = assert_norm (test_neg1_fwd = Return ())
@@ -138,10 +126,7 @@ let test_panic_fwd (b : bool) : result unit =
 
 (** [no_nested_borrows::test_copy_int] *)
 let test_copy_int_fwd : result unit =
-  begin match copy_int_fwd 0 with
-  | Fail e -> Fail e
-  | Return y -> if not (0 = y) then Fail Failure else Return ()
-  end
+  let* y = copy_int_fwd 0 in if not (0 = y) then Fail Failure else Return ()
 
 (** Unit test for [no_nested_borrows::test_copy_int] *)
 let _ = assert_norm (test_copy_int_fwd = Return ())
@@ -156,10 +141,8 @@ let is_cons_fwd (t : Type0) (l : list_t t) : result bool =
 (** [no_nested_borrows::test_is_cons] *)
 let test_is_cons_fwd : result unit =
   let l = ListNil in
-  begin match is_cons_fwd i32 (ListCons 0 l) with
-  | Fail e -> Fail e
-  | Return b -> if not b then Fail Failure else Return ()
-  end
+  let* b = is_cons_fwd i32 (ListCons 0 l) in
+  if not b then Fail Failure else Return ()
 
 (** Unit test for [no_nested_borrows::test_is_cons] *)
 let _ = assert_norm (test_is_cons_fwd = Return ())
@@ -174,11 +157,9 @@ let split_list_fwd (t : Type0) (l : list_t t) : result (t & (list_t t)) =
 (** [no_nested_borrows::test_split_list] *)
 let test_split_list_fwd : result unit =
   let l = ListNil in
-  begin match split_list_fwd i32 (ListCons 0 l) with
-  | Fail e -> Fail e
-  | Return p ->
-    let (hd, _) = p in if not (hd = 0) then Fail Failure else Return ()
-  end
+  let* p = split_list_fwd i32 (ListCons 0 l) in
+  let (hd, _) = p in
+  if not (hd = 0) then Fail Failure else Return ()
 
 (** Unit test for [no_nested_borrows::test_split_list] *)
 let _ = assert_norm (test_split_list_fwd = Return ())
@@ -194,24 +175,15 @@ let choose_back
 
 (** [no_nested_borrows::choose_test] *)
 let choose_test_fwd : result unit =
-  begin match choose_fwd i32 true 0 0 with
-  | Fail e -> Fail e
-  | Return z ->
-    begin match i32_add z 1 with
-    | Fail e -> Fail e
-    | Return z0 ->
-      if not (z0 = 1)
-      then Fail Failure
-      else
-        begin match choose_back i32 true 0 0 z0 with
-        | Fail e -> Fail e
-        | Return (x, y) ->
-          if not (x = 1)
-          then Fail Failure
-          else if not (y = 0) then Fail Failure else Return ()
-        end
-    end
-  end
+  let* z = choose_fwd i32 true 0 0 in
+  let* z0 = i32_add z 1 in
+  if not (z0 = 1)
+  then Fail Failure
+  else begin
+    let* (x, y) = choose_back i32 true 0 0 z0 in
+    if not (x = 1)
+    then Fail Failure
+    else if not (y = 0) then Fail Failure else Return () end
 
 (** Unit test for [no_nested_borrows::choose_test] *)
 let _ = assert_norm (choose_test_fwd = Return ())
@@ -232,11 +204,7 @@ and tree_t (t : Type0) =
 (** [no_nested_borrows::list_length] *)
 let rec list_length_fwd (t : Type0) (l : list_t t) : result u32 =
   begin match l with
-  | ListCons x l1 ->
-    begin match list_length_fwd t l1 with
-    | Fail e -> Fail e
-    | Return i -> u32_add 1 i
-    end
+  | ListCons x l1 -> let* i = list_length_fwd t l1 in u32_add 1 i
   | ListNil -> Return 0
   end
 
@@ -246,11 +214,7 @@ let rec list_nth_shared_fwd (t : Type0) (l : list_t t) (i : u32) : result t =
   | ListCons x tl ->
     if i = 0
     then Return x
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 -> list_nth_shared_fwd t tl i0
-      end
+    else begin let* i0 = u32_sub i 1 in list_nth_shared_fwd t tl i0 end
   | ListNil -> Fail Failure
   end
 
@@ -260,11 +224,7 @@ let rec list_nth_mut_fwd (t : Type0) (l : list_t t) (i : u32) : result t =
   | ListCons x tl ->
     if i = 0
     then Return x
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 -> list_nth_mut_fwd t tl i0
-      end
+    else begin let* i0 = u32_sub i 1 in list_nth_mut_fwd t tl i0 end
   | ListNil -> Fail Failure
   end
 
@@ -275,15 +235,10 @@ let rec list_nth_mut_back
   | ListCons x tl ->
     if i = 0
     then Return (ListCons ret tl)
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 ->
-        begin match list_nth_mut_back t tl i0 ret with
-        | Fail e -> Fail e
-        | Return tl0 -> Return (ListCons x tl0)
-        end
-      end
+    else begin
+      let* i0 = u32_sub i 1 in
+      let* tl0 = list_nth_mut_back t tl i0 ret in
+      Return (ListCons x tl0) end
   | ListNil -> Fail Failure
   end
 
@@ -305,57 +260,34 @@ let test_list_functions_fwd : result unit =
   let l = ListNil in
   let l0 = ListCons 2 l in
   let l1 = ListCons 1 l0 in
-  begin match list_length_fwd i32 (ListCons 0 l1) with
-  | Fail e -> Fail e
-  | Return i ->
-    if not (i = 3)
+  let* i = list_length_fwd i32 (ListCons 0 l1) in
+  if not (i = 3)
+  then Fail Failure
+  else begin
+    let* i0 = list_nth_shared_fwd i32 (ListCons 0 l1) 0 in
+    if not (i0 = 0)
     then Fail Failure
-    else
-      begin match list_nth_shared_fwd i32 (ListCons 0 l1) 0 with
-      | Fail e -> Fail e
-      | Return i0 ->
-        if not (i0 = 0)
+    else begin
+      let* i1 = list_nth_shared_fwd i32 (ListCons 0 l1) 1 in
+      if not (i1 = 1)
+      then Fail Failure
+      else begin
+        let* i2 = list_nth_shared_fwd i32 (ListCons 0 l1) 2 in
+        if not (i2 = 2)
         then Fail Failure
-        else
-          begin match list_nth_shared_fwd i32 (ListCons 0 l1) 1 with
-          | Fail e -> Fail e
-          | Return i1 ->
-            if not (i1 = 1)
+        else begin
+          let* ls = list_nth_mut_back i32 (ListCons 0 l1) 1 3 in
+          let* i3 = list_nth_shared_fwd i32 ls 0 in
+          if not (i3 = 0)
+          then Fail Failure
+          else begin
+            let* i4 = list_nth_shared_fwd i32 ls 1 in
+            if not (i4 = 3)
             then Fail Failure
-            else
-              begin match list_nth_shared_fwd i32 (ListCons 0 l1) 2 with
-              | Fail e -> Fail e
-              | Return i2 ->
-                if not (i2 = 2)
-                then Fail Failure
-                else
-                  begin match list_nth_mut_back i32 (ListCons 0 l1) 1 3 with
-                  | Fail e -> Fail e
-                  | Return ls ->
-                    begin match list_nth_shared_fwd i32 ls 0 with
-                    | Fail e -> Fail e
-                    | Return i3 ->
-                      if not (i3 = 0)
-                      then Fail Failure
-                      else
-                        begin match list_nth_shared_fwd i32 ls 1 with
-                        | Fail e -> Fail e
-                        | Return i4 ->
-                          if not (i4 = 3)
-                          then Fail Failure
-                          else
-                            begin match list_nth_shared_fwd i32 ls 2 with
-                            | Fail e -> Fail e
-                            | Return i5 ->
-                              if not (i5 = 2) then Fail Failure else Return ()
-                            end
-                        end
-                    end
-                  end
-              end
-          end
-      end
-  end
+            else begin
+              let* i5 = list_nth_shared_fwd i32 ls 2 in
+              if not (i5 = 2) then Fail Failure else Return () end end end end
+      end end
 
 (** Unit test for [no_nested_borrows::test_list_functions] *)
 let _ = assert_norm (test_list_functions_fwd = Return ())
@@ -433,37 +365,25 @@ let new_pair1_fwd : result (struct_with_pair_t u32 u32) =
 
 (** [no_nested_borrows::test_constants] *)
 let test_constants_fwd : result unit =
-  begin match new_tuple1_fwd with
-  | Fail e -> Fail e
-  | Return swt ->
-    let (i, _) = swt.struct_with_tuple_p in
-    if not (i = 1)
+  let* swt = new_tuple1_fwd in
+  let (i, _) = swt.struct_with_tuple_p in
+  if not (i = 1)
+  then Fail Failure
+  else begin
+    let* swt0 = new_tuple2_fwd in
+    let (i0, _) = swt0.struct_with_tuple_p in
+    if not (i0 = 1)
     then Fail Failure
-    else
-      begin match new_tuple2_fwd with
-      | Fail e -> Fail e
-      | Return swt0 ->
-        let (i0, _) = swt0.struct_with_tuple_p in
-        if not (i0 = 1)
+    else begin
+      let* swt1 = new_tuple3_fwd in
+      let (i1, _) = swt1.struct_with_tuple_p in
+      if not (i1 = 1)
+      then Fail Failure
+      else begin
+        let* swp = new_pair1_fwd in
+        if not (swp.struct_with_pair_p.pair_x = 1)
         then Fail Failure
-        else
-          begin match new_tuple3_fwd with
-          | Fail e -> Fail e
-          | Return swt1 ->
-            let (i1, _) = swt1.struct_with_tuple_p in
-            if not (i1 = 1)
-            then Fail Failure
-            else
-              begin match new_pair1_fwd with
-              | Fail e -> Fail e
-              | Return swp ->
-                if not (swp.struct_with_pair_p.pair_x = 1)
-                then Fail Failure
-                else Return ()
-              end
-          end
-      end
-  end
+        else Return () end end end
 
 (** Unit test for [no_nested_borrows::test_constants] *)
 let _ = assert_norm (test_constants_fwd = Return ())
diff --git a/tests/fstar/misc/Paper.fst b/tests/fstar/misc/Paper.fst
index 199ceb63..95f13f62 100644
--- a/tests/fstar/misc/Paper.fst
+++ b/tests/fstar/misc/Paper.fst
@@ -10,10 +10,8 @@ let ref_incr_fwd_back (x : i32) : result i32 = i32_add x 1
 
 (** [paper::test_incr] *)
 let test_incr_fwd : result unit =
-  begin match ref_incr_fwd_back 0 with
-  | Fail e -> Fail e
-  | Return x -> if not (x = 1) then Fail Failure else Return ()
-  end
+  let* x = ref_incr_fwd_back 0 in
+  if not (x = 1) then Fail Failure else Return ()
 
 (** Unit test for [paper::test_incr] *)
 let _ = assert_norm (test_incr_fwd = Return ())
@@ -29,24 +27,15 @@ let choose_back
 
 (** [paper::test_choose] *)
 let test_choose_fwd : result unit =
-  begin match choose_fwd i32 true 0 0 with
-  | Fail e -> Fail e
-  | Return z ->
-    begin match i32_add z 1 with
-    | Fail e -> Fail e
-    | Return z0 ->
-      if not (z0 = 1)
-      then Fail Failure
-      else
-        begin match choose_back i32 true 0 0 z0 with
-        | Fail e -> Fail e
-        | Return (x, y) ->
-          if not (x = 1)
-          then Fail Failure
-          else if not (y = 0) then Fail Failure else Return ()
-        end
-    end
-  end
+  let* z = choose_fwd i32 true 0 0 in
+  let* z0 = i32_add z 1 in
+  if not (z0 = 1)
+  then Fail Failure
+  else begin
+    let* (x, y) = choose_back i32 true 0 0 z0 in
+    if not (x = 1)
+    then Fail Failure
+    else if not (y = 0) then Fail Failure else Return () end
 
 (** Unit test for [paper::test_choose] *)
 let _ = assert_norm (test_choose_fwd = Return ())
@@ -62,11 +51,7 @@ let rec list_nth_mut_fwd (t : Type0) (l : list_t t) (i : u32) : result t =
   | ListCons x tl ->
     if i = 0
     then Return x
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 -> list_nth_mut_fwd t tl i0
-      end
+    else begin let* i0 = u32_sub i 1 in list_nth_mut_fwd t tl i0 end
   | ListNil -> Fail Failure
   end
 
@@ -77,26 +62,17 @@ let rec list_nth_mut_back
   | ListCons x tl ->
     if i = 0
     then Return (ListCons ret tl)
-    else
-      begin match u32_sub i 1 with
-      | Fail e -> Fail e
-      | Return i0 ->
-        begin match list_nth_mut_back t tl i0 ret with
-        | Fail e -> Fail e
-        | Return tl0 -> Return (ListCons x tl0)
-        end
-      end
+    else begin
+      let* i0 = u32_sub i 1 in
+      let* tl0 = list_nth_mut_back t tl i0 ret in
+      Return (ListCons x tl0) end
   | ListNil -> Fail Failure
   end
 
 (** [paper::sum] *)
 let rec sum_fwd (l : list_t i32) : result i32 =
   begin match l with
-  | ListCons x tl ->
-    begin match sum_fwd tl with
-    | Fail e -> Fail e
-    | Return i -> i32_add x i
-    end
+  | ListCons x tl -> let* i = sum_fwd tl in i32_add x i
   | ListNil -> Return 0
   end
 
@@ -105,22 +81,11 @@ let test_nth_fwd : result unit =
   let l = ListNil in
   let l0 = ListCons 3 l in
   let l1 = ListCons 2 l0 in
-  begin match list_nth_mut_fwd i32 (ListCons 1 l1) 2 with
-  | Fail e -> Fail e
-  | Return x ->
-    begin match i32_add x 1 with
-    | Fail e -> Fail e
-    | Return x0 ->
-      begin match list_nth_mut_back i32 (ListCons 1 l1) 2 x0 with
-      | Fail e -> Fail e
-      | Return l2 ->
-        begin match sum_fwd l2 with
-        | Fail e -> Fail e
-        | Return i -> if not (i = 7) then Fail Failure else Return ()
-        end
-      end
-    end
-  end
+  let* x = list_nth_mut_fwd i32 (ListCons 1 l1) 2 in
+  let* x0 = i32_add x 1 in
+  let* l2 = list_nth_mut_back i32 (ListCons 1 l1) 2 x0 in
+  let* i = sum_fwd l2 in
+  if not (i = 7) then Fail Failure else Return ()
 
 (** Unit test for [paper::test_nth] *)
 let _ = assert_norm (test_nth_fwd = Return ())
@@ -128,16 +93,8 @@ let _ = assert_norm (test_nth_fwd = Return ())
 (** [paper::call_choose] *)
 let call_choose_fwd (p : (u32 & u32)) : result u32 =
   let (px, py) = p in
-  begin match choose_fwd u32 true px py with
-  | Fail e -> Fail e
-  | Return pz ->
-    begin match u32_add pz 1 with
-    | Fail e -> Fail e
-    | Return pz0 ->
-      begin match choose_back u32 true px py pz0 with
-      | Fail e -> Fail e
-      | Return (px0, _) -> Return px0
-      end
-    end
-  end
+  let* pz = choose_fwd u32 true px py in
+  let* pz0 = u32_add pz 1 in
+  let* (px0, _) = choose_back u32 true px py pz0 in
+  Return px0
 
diff --git a/tests/fstar/misc/PoloniusList.fst b/tests/fstar/misc/PoloniusList.fst
index 12618dbb..db0dc0d5 100644
--- a/tests/fstar/misc/PoloniusList.fst
+++ b/tests/fstar/misc/PoloniusList.fst
@@ -25,11 +25,8 @@ let rec get_list_at_x_back
   | ListCons hd tl ->
     if hd = x
     then Return ret
-    else
-      begin match get_list_at_x_back tl x ret with
-      | Fail e -> Fail e
-      | Return tl0 -> Return (ListCons hd tl0)
-      end
+    else begin
+      let* tl0 = get_list_at_x_back tl x ret in Return (ListCons hd tl0) end
   | ListNil -> Return ret
   end
 
diff --git a/tests/fstar/misc/Primitives.fst b/tests/fstar/misc/Primitives.fst
index bf0f9078..98a696b6 100644
--- a/tests/fstar/misc/Primitives.fst
+++ b/tests/fstar/misc/Primitives.fst
@@ -35,7 +35,9 @@ unfold let return (#a : Type0) (x : a) : result a = Return x
 // let* x = y in
 // ...
 // ```
-unfold let (let*) (#a #b : Type0) (m: result a) (f: a -> result b) : result b =
+unfold let (let*) (#a #b : Type0) (m: result a)
+  (f: (x:a) -> Pure (result b) (requires (m == Return x)) (ensures fun _ -> True)) :
+  result b =
   match m with
   | Return x -> f x
   | Fail e   -> Fail e