From 97604d14f467458240732a4c0a733d381d72fbbe Mon Sep 17 00:00:00 2001
From: Son Ho
Date: Thu, 11 May 2023 12:34:14 +0200
Subject: Start working on more general fixed-point combinators

---
 backends/hol4/divDefProto2Script.sml | 678 +++++++++++++++++++++++++++++++++++
 1 file changed, 678 insertions(+)
 create mode 100644 backends/hol4/divDefProto2Script.sml

(limited to 'backends/hol4/divDefProto2Script.sml')

diff --git a/backends/hol4/divDefProto2Script.sml b/backends/hol4/divDefProto2Script.sml
new file mode 100644
index 00000000..985b930f
--- /dev/null
+++ b/backends/hol4/divDefProto2Script.sml
@@ -0,0 +1,678 @@
+(* Prototype: divDefLib but with general combinators *)
+
+open HolKernel boolLib bossLib Parse
+open boolTheory arithmeticTheory integerTheory intLib listTheory stringTheory
+
+open primitivesArithTheory primitivesBaseTacLib ilistTheory primitivesTheory
+open primitivesLib
+
+val _ = new_theory "divDefProto2"
+
+
+(*
+ * Test with a general validity predicate.
+ *
+ * TODO: this works! Cleanup.
+ *)
+val fix_fuel_def = Define ‘
+  (fix_fuel (0 : num) (f : ('a -> 'a result) -> 'a -> 'a result) (x : 'a) : 'a result = Diverge) ∧
+  (fix_fuel (SUC n) (f : ('a -> 'a result) -> 'a -> 'a result) (x : 'a) : 'a result = f (fix_fuel n f) x)
+’
+
+val fix_fuel_P_def = Define ‘
+  fix_fuel_P f x n = ~(is_diverge (fix_fuel n f x))
+’
+
+val fix_def = Define ‘
+  fix (f : ('a -> 'a result) -> 'a -> 'a result) (x : 'a) : 'a result =
+    if (∃ n. fix_fuel_P f x n) then fix_fuel ($LEAST (fix_fuel_P f x)) f x else Diverge
+’
+
+val is_valid_fp_body_def = Define ‘
+  (is_valid_fp_body (0 : num) (f : ('a -> 'a result) -> 'a -> 'a result) = F) ∧
+  
+  (is_valid_fp_body (SUC n) (f : ('a -> 'a result) -> 'a -> 'a result) =
+    ∀x. (∀g h. f g x = f h x) ∨
+         (∃ h y. is_valid_fp_body n h ∧
+                  ∀g. f g x = do z <- g y; h g z od))
+’
+
+(* Auxiliary lemma.
+   We generalize the goal of fix_fuel_mono in the case the fuel is non-empty
+   (this allows us to unfold definitions like ‘fix_fuel’ once, and reveal
+   a first intermediate function).
+
+   Important: the structure of the proof is induction over ‘n’ then ‘N’.
+ *)
+Theorem fix_fuel_mono_aux:
+  ∀n.
+    ∀N M g f. is_valid_fp_body M f ⇒
+      is_valid_fp_body N g ⇒
+      ∀x. ~(is_diverge (g (fix_fuel n f) x)) ⇒
+       ∀m. n ≤ m ⇒
+         g (fix_fuel n f) x = g (fix_fuel m f) x
+Proof
+  Induct_on ‘n’ >>
+  Induct_on ‘N’ >- fs [is_valid_fp_body_def]
+  >-(  
+    rw [] >>
+    fs [is_valid_fp_body_def, is_diverge_def] >>
+    first_x_assum (qspec_assume ‘x’) >>
+    rw []
+    >-((* Case 1: the continuation doesn't matter *) fs []) >>
+    (* Case 2: the continuation *does* matter (i.e., there is a recursive call *)
+    (* Instantiate the validity property with the different continuations *)
+    first_assum (qspec_assume ‘fix_fuel n f’) >>
+    first_assum (qspec_assume ‘fix_fuel n' f’) >>
+    fs [] >>
+    ntac 3 (pop_assum ignore_tac) >>
+    fs [bind_def] >>
+    fs [fix_fuel_def])
+  >-(fs [is_valid_fp_body_def]) >>
+  rw [] >>
+  qpat_assum ‘is_valid_fp_body (SUC N) g’ mp_tac >>
+  pure_rewrite_tac [is_valid_fp_body_def] >>
+  fs [is_diverge_def] >>
+  rw [] >>
+  first_x_assum (qspec_assume ‘x’) >>
+  rw []
+  >-((* Case 1: the continuation doesn't matter *) fs []) >>
+  (* Case 2: the continuation *does* matter (i.e., there is a recursive call *)
+  (* Use the validity property with the different continuations *)
+  fs [] >> pop_assum ignore_tac >>
+  fs [bind_def, fix_fuel_def] >>
+  Cases_on ‘m’ >- int_tac >>
+  fs [fix_fuel_def] >>
+  (* *)
+  last_x_assum (qspecl_assume [‘M’, ‘M’, ‘f’, ‘f’]) >>
+  gvs [] >>
+  first_x_assum (qspec_assume ‘y’) >>
+  Cases_on ‘f (fix_fuel n f) y’ >> fs [] >>
+  first_x_assum (qspec_assume ‘n'’) >> gvs [] >> Cases_on ‘f (fix_fuel n' f) y’ >> fs [] >>
+  (* *)  
+  first_assum (qspecl_assume [‘M’, ‘h’, ‘f’]) >>
+  gvs []
+QED
+
+Theorem fix_fuel_mono:
+  ∀N f. is_valid_fp_body N f ⇒
+    ∀n x. fix_fuel_P f x n ⇒
+     ∀ m. n ≤ m ⇒
+       fix_fuel n f x = fix_fuel m f x
+Proof
+  rw [] >>
+  Cases_on ‘n’
+  >-(fs [fix_fuel_P_def, is_diverge_def, fix_fuel_def]) >>
+  fs [fix_fuel_P_def, fix_fuel_def] >> rw [] >>
+  qspecl_assume [‘n'’, ‘N’, ‘N’, ‘f’, ‘f’] fix_fuel_mono_aux >>
+  Cases_on ‘m’ >- fs [] >>
+  gvs [fix_fuel_def]
+QED
+
+(* TODO: remove? *)
+Theorem fix_fuel_mono_least:
+  ∀N f. is_valid_fp_body N f ⇒
+    ∀n x. fix_fuel_P f x n ⇒
+      fix_fuel n f x = fix_fuel ($LEAST (fix_fuel_P f x)) f x
+Proof
+  rw [] >>
+  pure_once_rewrite_tac [EQ_SYM_EQ] >>
+  irule fix_fuel_mono >> fs [] >>
+  (* Use the "fundamental" property about $LEAST *)
+  qspec_assume ‘fix_fuel_P f x’ whileTheory.LEAST_EXISTS_IMP >>
+  (* Prove the premise *)
+  pop_assum sg_premise_tac >- metis_tac [] >> fs [] >>
+  conj_tac
+  >- (spose_not_then assume_tac >> fs [not_le_eq_gt]) >>
+  metis_tac []
+QED
+
+Theorem fix_fuel_eq_fix:
+  ∀N f. is_valid_fp_body N f ⇒
+    ∀n x. fix_fuel_P f x n ⇒
+      fix_fuel n f x = fix f x
+Proof
+  fs [fix_def] >>
+  rw [] >>
+  imp_res_tac fix_fuel_mono_least >>
+  fs [fix_fuel_P_def, is_diverge_def] >>
+  case_tac >> fs []
+QED
+
+Theorem fix_fuel_P_least:
+  ∀f n x. fix_fuel n f x ≠ Diverge ⇒ fix_fuel_P f x ($LEAST (fix_fuel_P f x))
+Proof
+  rw [] >>
+  qspec_assume ‘fix_fuel_P f x’ whileTheory.LEAST_EXISTS_IMP >>
+  (* Prove the premise *)
+  pop_assum sg_premise_tac
+  >-(fs [fix_fuel_P_def, is_diverge_def] >> qexists ‘n’ >> fs [] >> case_tac >> fs []) >>
+  rw []
+QED
+
+(* If ‘g (fix f) x’ doesn't diverge, we can exhibit some fuel *)
+Theorem fix_not_diverge_implies_fix_fuel_aux:
+  ∀N M g f. is_valid_fp_body M f ⇒
+    is_valid_fp_body N g ⇒
+     ∀x. g (fix f) x ≠ Diverge ⇒
+       ∃n. g (fix f) x = g (fix_fuel n f) x ∧
+         ∀m. n ≤ m ⇒ g (fix_fuel m f) x = g (fix_fuel n f) x
+Proof
+  Induct_on ‘N’
+  >-(fs [is_valid_fp_body_def]) >>
+  rw [is_valid_fp_body_def] >>
+  first_x_assum (qspec_assume ‘x’) >> rw []
+  >-(first_assum (qspecl_assume [‘fix f’, ‘fix_fuel 0 f’]) >> fs []) >>
+  (* Use the validity hypothesis *)
+  fs [] >> pop_assum ignore_tac >>
+  (* Use the induction hypothesis *)
+  last_x_assum (qspecl_assume [‘M’, ‘h’, ‘f’]) >> gvs [] >>
+  (* Case disjunction on ‘fix f ÿ’*)
+  Cases_on ‘fix f y’ >> fs [bind_def] >~ [‘fix f y = Fail _’]
+  >-(
+    (* Fail case: easy, the call to ‘h’ is ignored *)
+    fs [fix_def] >> pop_assum mp_tac >> rw [] >>
+    qexists ‘$LEAST (fix_fuel_P f y)’ >>
+    fs [] >>
+    (* Use the monotonicity property for ‘f’ *)
+    rw [] >>
+    qspecl_assume [‘M’, ‘f’] fix_fuel_mono >> gvs [] >>
+    first_x_assum (qspecl_assume [‘$LEAST (fix_fuel_P f y)’, ‘y’]) >> gvs [] >>
+    fs [fix_fuel_P_def, is_diverge_def] >> gvs [] >>
+    first_x_assum (qspecl_assume [‘m’]) >> gvs [] >>
+    first_x_assum (fn th => assume_tac (GSYM th)) >> fs []
+    ) >>
+  (* Return case: we must take the maximum of the fuel for ‘f’ and ‘h’, and use
+     the monotonicity property *)
+  fs [fix_def] >> pop_assum mp_tac >> rw [] >>
+  first_x_assum (qspec_assume ‘a’) >> gvs [] >>
+  qexists ‘MAX ($LEAST (fix_fuel_P f y)) n'’ >> fs [] >>
+  (* Use the monotonicity properties *)
+  (* Instantiate the Monotonicity property for ‘f’ (the induction hypothesis gives
+     the one for ‘h’) *)
+  qspecl_assume [‘M’, ‘f’] fix_fuel_mono >> gvs [] >>
+  first_x_assum (qspecl_assume [‘$LEAST (fix_fuel_P f y)’, ‘y’]) >> gvs [] >>
+  fs [fix_fuel_P_def, is_diverge_def] >> gvs [] >>
+  first_x_assum (qspecl_assume [‘MAX ($LEAST (fix_fuel_P f y)) n'’]) >> gvs [] >>
+  first_x_assum (fn th => assume_tac (GSYM th)) >> fs [] >>
+  (* Prove the monotonicity property for ‘do z <- fix f y; h (fix f) z’ *)
+  rw [] >>
+  (* First, one of the ‘fix_fuel ... f y’ doesn't use the proper fuel *)
+  sg ‘fix_fuel ($LEAST (fix_fuel_P f y)) f y = Return a’
+  >-(
+    qspecl_assume [‘f’, ‘MAX ($LEAST (fix_fuel_P f y)) n'’, ‘y’] fix_fuel_P_least >>
+    gvs [fix_fuel_P_def, is_diverge_def] >>
+    Cases_on ‘fix_fuel ($LEAST (fix_fuel_P f y)) f y’ >> fs [] >>
+    (* Use the monotonicity property - there are two goals here *)
+    qspecl_assume [‘M’, ‘f’] fix_fuel_mono >> gvs [] >>
+    first_x_assum (qspecl_assume [‘$LEAST (fix_fuel_P f y)’, ‘y’]) >> gvs [] >>
+    fs [fix_fuel_P_def, is_diverge_def] >> gvs [] >>
+    first_x_assum (qspecl_assume [‘MAX ($LEAST (fix_fuel_P f y)) n'’]) >> gvs []) >>
+  (* Instantiate the monotonicity property for ‘f’ *)
+  qspecl_assume [‘M’, ‘f’] fix_fuel_mono >> gvs [] >>
+  first_x_assum (qspecl_assume [‘$LEAST (fix_fuel_P f y)’, ‘y’]) >> gvs [] >>
+  gvs [fix_fuel_P_def, is_diverge_def] >> gvs [] >>
+  first_x_assum (qspecl_assume [‘m’]) >> gvs [] >>
+  first_x_assum (fn th => assume_tac (GSYM th)) >> fs []
+QED
+
+(* If ‘g (fix f) x’ doesn't diverge, we can exhibit some fuel *)
+Theorem fix_not_diverge_implies_fix_fuel:
+  ∀N f. is_valid_fp_body N f ⇒
+     ∀x. f (fix f) x ≠ Diverge ⇒
+       ∃n. f (fix f) x = f (fix_fuel n f) x
+Proof
+  metis_tac [fix_not_diverge_implies_fix_fuel_aux]
+QED    
+
+Theorem fix_fixed_diverges:
+  ∀N f. is_valid_fp_body N f ⇒ ∀x. ~(∃ n. fix_fuel_P f x n) ⇒ fix f x = f (fix f) x
+Proof
+  (* We do the proof by contraposition: if ‘f (fix f) x’ doesn't diverge, we
+     can exhibit some fuel (lemma [fix_not_diverge_implies_fix_fuel]) *)
+  rw [fix_def] >>
+  imp_res_tac fix_not_diverge_implies_fix_fuel >>
+  pop_assum (qspec_assume ‘x’) >>
+  fs [fix_fuel_P_def, is_diverge_def] >>
+  (* Case analysis: we have to prove that the ‘Return’ and ‘Fail’ cases lead
+     to a contradiction *)
+  Cases_on ‘f (fix f) x’ >> gvs [] >>
+  first_x_assum (qspec_assume ‘SUC n’) >> fs [fix_fuel_def] >>
+  pop_assum mp_tac >> case_tac >> fs []
+QED
+
+(* If ‘g (fix_fuel n f) x’ doesn't diverge, then it is equal to ‘g (fix f) x’ *)
+Theorem fix_fuel_not_diverge_eq_fix_aux:
+  ∀N M g f. is_valid_fp_body M f ⇒
+    is_valid_fp_body N g ⇒
+     ∀n x. g (fix_fuel n f) x ≠ Diverge ⇒
+       g (fix f) x = g (fix_fuel n f) x
+Proof
+  Induct_on ‘N’
+  >-(fs [is_valid_fp_body_def]) >>
+  rw [is_valid_fp_body_def] >>
+  first_x_assum (qspec_assume ‘x’) >> rw []
+  >-(first_assum (qspecl_assume [‘fix f’, ‘fix_fuel 0 f’]) >> fs []) >>
+  (* Use the validity hypothesis *)
+  fs [] >> pop_assum ignore_tac >>
+  (* For ‘fix f y = fix_fuel n f y’: use the monotonicity property *)
+  sg ‘fix_fuel_P f y n’
+  >-(Cases_on ‘fix_fuel n f y’ >> fs [fix_fuel_P_def, is_diverge_def, bind_def]) >>
+  sg ‘fix f y = fix_fuel n f y’ >-(metis_tac [fix_fuel_eq_fix])>>
+  (* Case disjunction on the call to ‘f’ *)
+  Cases_on ‘fix_fuel n f y’ >> gvs [bind_def] >>
+  (* We have to prove that: ‘h (fix f) a = h (fix_fuel n f) a’: use the induction hypothesis *)
+  metis_tac []
+QED
+
+Theorem fix_fuel_not_diverge_eq_fix:
+  ∀N f. is_valid_fp_body N f ⇒
+     ∀n x. f (fix_fuel n f) x ≠ Diverge ⇒
+       f (fix f) x = f (fix_fuel n f) x
+Proof
+  metis_tac [fix_fuel_not_diverge_eq_fix_aux]
+QED
+
+Theorem fix_fixed_terminates:
+  ∀N f. is_valid_fp_body N f ⇒ ∀x n. fix_fuel_P f x n ⇒ fix f x = f (fix f) x
+Proof
+  (* The proof simply uses the lemma [fix_fuel_not_diverge_eq_fix] *)
+  rw [fix_fuel_P_def, is_diverge_def, fix_def] >> case_tac >> fs [] >>
+  (* We can prove that ‘fix_fuel ($LEAST ...) f x ≠ Diverge’ *)
+  qspecl_assume [‘f’, ‘n’, ‘x’] fix_fuel_P_least >>
+  pop_assum sg_premise_tac >-(Cases_on ‘fix_fuel n f x’ >> fs []) >>
+  fs [fix_fuel_P_def, is_diverge_def] >>
+  (* *)
+  Cases_on ‘($LEAST (fix_fuel_P f x))’ >> fs [fix_fuel_def] >>
+  irule (GSYM fix_fuel_not_diverge_eq_fix) >>
+  Cases_on ‘f (fix_fuel n'' f) x’ >> fs [] >> metis_tac []
+QED
+
+(*
+Type ft = ``: 'a -> ('a result + (num # num # 'a))``
+
+val fix_fuel_def = Define ‘
+  (fix_fuel (0 : num) (fs : ('a ft) list)
+            (i : num) (x : 'a) : 'a result = Diverge) ∧
+  
+  (fix_fuel (SUC n) fs i x =
+    case EL i fs x of
+    | INL r => r
+    | INR (j, k, y) =>
+      case fix_fuel n fs j y of
+      | Fail e => Fail e
+      | Diverge => Diverge
+      | Return z =>
+        fix_fuel n fs k z)
+’
+
+val fix_fuel_P_def = Define ‘
+  fix_fuel_P fs i x n = ~(is_diverge (fix_fuel n fs i x))
+’
+
+val fix_def = Define ‘
+  fix (fs : ('a ft) list) (i : num) (x : 'a) : 'a result =
+    if (∃ n. fix_fuel_P fs i x n)
+    then fix_fuel ($LEAST (fix_fuel_P fs i x)) fs i x
+    else Diverge
+’
+
+Theorem fix_fuel_mono:
+  ∀f. is_valid_fp_body f ⇒
+    ∀n x. fix_fuel_P f x n ⇒
+     ∀ m. n ≤ m ⇒
+       fix_fuel n f x = fix_fuel m f x
+Proof
+  ntac 2 strip_tac >>
+  Induct_on ‘n’ >> rpt strip_tac
+  >- (fs [fix_fuel_P_def, is_diverge_def, fix_fuel_def]) >>
+  Cases_on ‘m’ >- int_tac >> fs [] >>
+  fs [is_valid_fp_body_def] >>
+  fs [fix_fuel_P_def, is_diverge_def, fix_fuel_def] >>
+
+  (*(* Use the validity property *)
+  last_assum (qspec_assume ‘x’) >> (* TODO: consume? *) *)
+
+  (*pop_assum ignore_tac >> (* TODO: not sure *) *)
+  Induct_on ‘N’ >- fs [eval_ftree_def] >>
+  rw [] >>
+  rw [eval_ftree_def] >>
+  Cases_on ‘h x’ >> fs [] >>
+  Cases_on ‘y’ >> fs [] >>
+  Cases_on ‘y'’ >> fs [] >>
+  
+  last_assum (qspec_assume ‘q’) >>
+  Cases_on ‘fix_fuel n f q’ >> fs [] >>
+  
+  Cases_on ‘N’ >> fs [eval_ftree_def] >>
+  
+  Cases_on ‘y’ >> fs [] >>
+  Cases_on ‘y'’ >> fs [] >>
+  rw [] >>
+  (* This makes a case disjunction on the validity property *)
+  rw []
+  >-((* Case 1: the continuation doesn't matter *) fs []) >>
+  (* Case 2: the continuation *does* matter (i.e., there is a recursive call *)
+  (* Instantiate the validity property with the different continuations *)
+  first_assum (qspec_assume ‘fix_fuel n f’) >>
+  first_assum (qspec_assume ‘fix_fuel n' f’) >>
+  last_assum (qspec_assume ‘y’) >>
+  fs []
+QED
+
+
+
+Type ft = ``: ('a -> 'a result) -> 'a -> ('a result + (num # 'a))``
+
+val fix_fuel_def = Define ‘
+  (fix_fuel (0 : num) (fs : ('a ft) list) (g : 'a -> 'a result)
+            (i : num) (x : 'a) : 'a result = Diverge) ∧
+  
+  (fix_fuel (SUC n) fs g i x =
+    case EL i fs g x of
+    | INL r => r
+    | INR (j, y) =>
+      case g y of
+      | Fail e => Fail e
+      | Diverge => Diverge
+      | Return z => fix_fuel n fs g j z)
+’
+
+val fix_fuel_def = Define ‘
+  (fix_fuel (0 : num) (fs : ('a ft) list) g (i : num) (x : 'a) : 'a result = Diverge) ∧
+  
+  (fix_fuel (SUC n) fs g i x =
+    case EL i fs x of
+    | INL r => r
+    | INR (j, y) =>
+      case g y of
+      | Fail e => Fail e
+      | Diverge => Diverge
+      | Return z => fix_fuel n fs g j z)
+’
+
+val fix_fuel_def = Define ‘
+  (fix_fuel (0 : num) (f : ('a -> 'a result) -> 'a -> 'a result) (x : 'a) : 'a result = Diverge) ∧
+  (fix_fuel (SUC n) (f : ('a -> 'a result) -> 'a -> 'a result) (x : 'a) : 'a result = f (fix_fuel n f) x)
+’
+
+val fix_fuel_P_def = Define ‘
+  fix_fuel_P f x n = ~(is_diverge (fix_fuel n f x))
+’
+
+val fix_def = Define ‘
+  fix (f : ('a -> 'a result) -> 'a -> 'a result) (x : 'a) : 'a result =
+    if (∃ n. fix_fuel_P f x n) then fix_fuel ($LEAST (fix_fuel_P f x)) f x else Diverge
+’
+
+(*Datatype:
+  ftree = Rec (('a -> ('a result + ('a # num))) # ftree list) | NRec ('a -> 'a result)
+End
+
+Type frtree = ``: ('b -> ('b result + ('a # num))) list``
+Type ftree = “: ('a, 'b) frtree # ('b result + ('a # num))”
+*)
+
+val eval_ftree_def = Define ‘
+  (eval_ftree 0 (g : 'a -> 'a result)
+    (fs : ('a -> ('a result + ('a # num))) list) x = Diverge) ∧
+
+  (eval_ftree (SUC n) g fs x =
+    case x of
+    | INL r => r
+    | INR (y, i) =>
+      case g y of
+      | Fail e => Fail e
+      | Diverge => Diverge
+      | Return z =>
+        let f = EL i fs in
+        eval_ftree n g fs (f z))
+’
+
+Theorem fix_fuel_mono:
+  ∀N fs i.
+  let f = (\g x. eval_ftree N g fs (INR (x, i))) in
+  ∀n x. fix_fuel_P f x n ⇒
+  ∀ m. n ≤ m ⇒ fix_fuel n f x = fix_fuel m f x
+Proof
+  Induct_on ‘N’
+  >-(
+    fs [eval_ftree_def] >>
+
+  ntac 2 strip_tac >>
+  Induct_on ‘n’ >> rpt strip_tac
+  >- (fs [fix_fuel_P_def, is_diverge_def, fix_fuel_def]) >>
+  Cases_on ‘m’ >- int_tac >> fs [] >>
+  fs [is_valid_fp_body_def] >>
+  fs [fix_fuel_P_def, is_diverge_def, fix_fuel_def] >>
+
+
+val is_valid_fp_body_def = Define ‘
+  is_valid_fp_body (f : ('a -> 'b result) -> 'a -> 'b result) =
+    (∃N ft. ∀x g. f g x = eval_ftree N g ft (x, i))
+’
+
+val eval_ftree_def = Define ‘
+  (eval_ftree 0 (g : 'a -> 'b result)
+    (fs : ('b -> ('b result + ('a # num))) list, x : 'b result + ('a # num)) = Diverge) ∧
+  
+  (eval_ftree (SUC n) g (fs, x) =
+    case x of
+    | INL r => r
+    | INR (y, i) =>
+      case g y of
+      | Fail e => Fail e
+      | Diverge => Diverge
+      | Return z =>
+        let f = EL i fs in
+        eval_ftree n g (fs, f z))
+’
+
+val is_valid_fp_body_def = Define ‘
+  is_valid_fp_body (f : ('a -> 'b result) -> 'a -> 'b result) =
+    (∃N ft h. ∀x g. f g x = eval_ftree N g (ft, h x))
+’
+
+Theorem fix_fuel_mono:
+  let f = (\x. eval_ftree N g (ft, h x)) in
+  ∀n x. fix_fuel_P f x n ⇒
+   ∀ m. n ≤ m ⇒ fix_fuel n f x = fix_fuel m f x
+Proof
+
+
+Theorem fix_fuel_mono:
+  ∀f. is_valid_fp_body f ⇒
+    ∀n x. fix_fuel_P f x n ⇒
+     ∀ m. n ≤ m ⇒
+       fix_fuel n f x = fix_fuel m f x
+Proof
+  ntac 2 strip_tac >>
+  Induct_on ‘n’ >> rpt strip_tac
+  >- (fs [fix_fuel_P_def, is_diverge_def, fix_fuel_def]) >>
+  Cases_on ‘m’ >- int_tac >> fs [] >>
+  fs [is_valid_fp_body_def] >>
+  fs [fix_fuel_P_def, is_diverge_def, fix_fuel_def] >>
+
+  (*(* Use the validity property *)
+  last_assum (qspec_assume ‘x’) >> (* TODO: consume? *) *)
+
+  (*pop_assum ignore_tac >> (* TODO: not sure *) *)
+  Induct_on ‘N’ >- fs [eval_ftree_def] >>
+  rw [] >>
+  rw [eval_ftree_def] >>
+  Cases_on ‘h x’ >> fs [] >>
+  Cases_on ‘y’ >> fs [] >>
+  Cases_on ‘y'’ >> fs [] >>
+  
+  last_assum (qspec_assume ‘q’) >>
+  Cases_on ‘fix_fuel n f q’ >> fs [] >>
+  
+  Cases_on ‘N’ >> fs [eval_ftree_def] >>
+  
+  Cases_on ‘y’ >> fs [] >>
+  Cases_on ‘y'’ >> fs [] >>
+  rw [] >>
+  (* This makes a case disjunction on the validity property *)
+  rw []
+  >-((* Case 1: the continuation doesn't matter *) fs []) >>
+  (* Case 2: the continuation *does* matter (i.e., there is a recursive call *)
+  (* Instantiate the validity property with the different continuations *)
+  first_assum (qspec_assume ‘fix_fuel n f’) >>
+  first_assum (qspec_assume ‘fix_fuel n' f’) >>
+  last_assum (qspec_assume ‘y’) >>
+  fs []
+QED
+
+
+val length_ftree = “
+(
+  [
+    (\n. INL (return (1 + n)))
+  ],
+  (case ls of
+   | ListCons x tl =>
+     INR (tl, 0)
+   | ListNil => INL (return 0))
+) : ('a list_t, int) ftree
+”
+
+val eval_length_ftree = mk_icomb (“eval_ftree 1 g”, length_ftree)
+
+Theorem length_body_eq:
+  eval_ftree (SUC (SUC 0)) g
+  (
+    [
+      (\n. INL (Return (1 + n)))
+    ],
+    (case ls of
+     | ListCons x tl =>
+       INR (tl, 0)
+     | ListNil => INL (Return 0))
+  ) =
+  case ls of
+  | ListCons x tl =>
+    do
+      n <- g tl;
+      Return (1 + n)
+    od
+  | ListNil => Return 0
+Proof
+  fs [eval_ftree_def, bind_def] >>
+  Cases_on ‘ls’ >> fs []
+QED
+
+val eval_ftree_def = Define ‘
+  eval_ftree 0 (fs : ('a, 'b) ftree) (g : 'a -> 'b result) (x : 'b result + ('a # num)) = Diverge ∧
+  
+  eval_ftree (SUC n) fs g x =
+    case x of
+    | INL r => r
+    | INR (y, i) =>
+      case g y of
+      | Fail e => Fail e
+      | Diverge => Diverge
+      | Return z =>
+        let f = EL i fs in
+        eval_ftree n fs g (f z)
+’
+
+val length_body_funs_def = Define 
+
+“
+[
+  (\ls. case ls of
+   | ListCons x tl =>
+     INR (tl, 1)
+   | ListNil => INL (return 0)),
+  (\n. INL (return (1 + n)))
+]
+”
+
+
+
+“:('a, 'b) FT”
+
+Define 
+
+val nth_body = Define ‘
+  
+
+’
+
+“INL”
+“INR”
+
+“
+  Rec (
+    case ls of
+    | ListCons x tl =>
+      do
+        INR (tl, 0)
+      od
+    | ListNil => INL (return 0),
+    [NRec (\n. return (1 + n))])
+”
+
+“
+  case ls of
+  | ListCons x tl =>
+    if u32_to_int i = (0:int)
+    then (Return x)
+    else
+      do
+      i0 <- u32_sub i (int_to_u32 1);
+      y <- nth tl i0;
+      return y
+      od
+  | ListNil => Fail Failure
+”
+
+
+“:'a + 'b”
+“:'a # 'b”
+
+(*** Encoding of a function *)
+Datatype:
+  ('a, 'b) test = Return ('a -> 'b)
+End
+
+val tyax =
+      new_type_definition ("three",
+        Q.prove(`?p. (\(x,y). ~(x /\ y)) p`, cheat))
+
+val three_bij = define_new_type_bijections
+                      {name="three_tybij", ABS="abs3", REP="rep3", tyax=tyax}
+type_of “rep3”
+type_of “abs3”
+
+m “”
+
+                Q.EXISTS_TAC `(F,F)` THEN GEN_BETA_TAC THEN REWRITE_TAC []));
+
+“Return (\x. x)”
+
+Datatype:
+  ftree = Rec ('a -> ('a result + ('a # ftree))) | NRec ('a -> 'a result)
+End
+
+Datatype:
+  'a ftree = Rec ('a -> ('a result + ('a # ftree))) | NRec ('a -> 'a result)
+End
+
+Datatype:
+  ftree = Rec ('a -> ('a result + ('a # ftree))) | NRec ('a -> 'a result)
+End
+
+Datatype:
+  result = Return 'a | Fail error | Diverge
+End
+
+Type M = ``: 'a result``
+
+
+val fix_def = Define ‘
+  fix (f : ('a -> 'b result) -> 'a -> 'b result) (x : 'a) : 'b result =
+    if (∃ n. fix_fuel_P f x n) then fix_fuel ($LEAST (fix_fuel_P f x)) f x else Diverge
+’
+
+val _ = export_theory ()
-- 
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