Reorganize backends/lean/Base

4 files changed, 1298 insertions, 1102 deletions

diff --git a/backends/lean/Base/Diverge.lean b/backends/lean/Base/Diverge.lean
index c62e6dd5..c9a2eec2 100644
--- a/backends/lean/Base/Diverge.lean
+++ b/backends/lean/Base/Diverge.lean
@@ -3,1105 +3,5 @@ import Lean.Meta.Tactic.Simp
 import Init.Data.List.Basic
 import Mathlib.Tactic.RunCmd
 import Mathlib.Tactic.Linarith
-
-/-
-TODO:
-- we want an easier to use cases:
-  - keeps in the goal an equation of the shape: `t = case`
-  - if called on Prop terms, uses Classical.em
-  Actually, the cases from mathlib seems already quite powerful
-  (https://leanprover-community.github.io/mathlib_docs/tactics.html#cases)
-  For instance: cases h : e
-  Also: cases_matching
-- better split tactic
-- we need conversions to operate on the head of applications.
-  Actually, something like this works:
-  ```
-  conv at Hl =>
-    apply congr_fun
-    simp [fix_fuel_P]
-  ```
-  Maybe we need a rpt ... ; focus?
-- simplifier/rewriter have a strange behavior sometimes
--/
-
-
-/- TODO: this is very useful, but is there more? -/
-set_option profiler true
-set_option profiler.threshold 100
-
-namespace Diverge
-
-namespace Primitives
-/-! # Copy-pasting from Primitives to make the file self-contained -/
-
-inductive Error where
-   | assertionFailure: Error
-   | integerOverflow: Error
-   | divisionByZero: Error
-   | arrayOutOfBounds: Error
-   | maximumSizeExceeded: Error
-   | panic: Error
-deriving Repr, BEq
-
-open Error
-
-inductive Result (α : Type u) where
-  | ret (v: α): Result α
-  | fail (e: Error): Result α
-  | div
-deriving Repr, BEq
-
-open Result
-
-def bind (x: Result α) (f: α -> Result β) : Result β :=
-  match x with
-  | ret v  => f v 
-  | fail v => fail v
-  | div => div
-
-@[simp] theorem bind_ret (x : α) (f : α → Result β) : bind (.ret x) f = f x := by simp [bind]
-@[simp] theorem bind_fail (x : Error) (f : α → Result β) : bind (.fail x) f = .fail x := by simp [bind]
-@[simp] theorem bind_div (f : α → Result β) : bind .div f = .div := by simp [bind]
-
--- Allows using Result in do-blocks
-instance : Bind Result where
-  bind := bind
-
--- Allows using return x in do-blocks
-instance : Pure Result where
-  pure := fun x => ret x
-
-@[simp] theorem bind_tc_ret (x : α) (f : α → Result β) :
-  (do let y ← .ret x; f y) = f x := by simp [Bind.bind, bind]
-
-@[simp] theorem bind_tc_fail (x : Error) (f : α → Result β) :
-  (do let y ← fail x; f y) = fail x := by simp [Bind.bind, bind]
-
-@[simp] theorem bind_tc_div (f : α → Result β) :
-  (do let y ← div; f y) = div := by simp [Bind.bind, bind]
-
-def div? {α: Type} (r: Result α): Bool :=
-  match r with
-  | div => true
-  | ret _ | fail _ => false
-
-end Primitives
-
-namespace Fix
-
-  open Primitives
-  open Result
-
-  variable {a : Type} {b : a → Type}
-  variable {c d : Type}
-
-  /-! # The least fixed point definition and its properties -/
-
-  def least_p (p : Nat → Prop) (n : Nat) : Prop := p n ∧ (∀ m, m < n → ¬ p m)
-  noncomputable def least (p : Nat → Prop) : Nat :=
-    Classical.epsilon (least_p p)
-
-  -- Auxiliary theorem for [least_spec]: if there exists an `n` satisfying `p`,
-  -- there there exists a least `m` satisfying `p`.
-  theorem least_spec_aux (p : Nat → Prop) : ∀ (n : Nat), (hn : p n) → ∃ m, least_p p m := by
-    apply Nat.strongRec'
-    intros n hi hn
-    -- Case disjunction on: is n the smallest n satisfying p?
-    match Classical.em (∀ m, m < n → ¬ p m) with
-    | .inl hlt =>
-      -- Yes: trivial
-      exists n
-    | .inr hlt =>
-      simp at *
-      let ⟨ m, ⟨ hmlt, hm ⟩ ⟩ := hlt
-      have hi := hi m hmlt hm
-      apply hi
-
-  -- The specification of [least]: either `p` is never satisfied, or it is satisfied
-  -- by `least p` and no `n < least p` satisfies `p`.
-  theorem least_spec (p : Nat → Prop) : (∀ n, ¬ p n) ∨ (p (least p) ∧ ∀ n, n < least p → ¬ p n) := by
-    -- Case disjunction on the existence of an `n` which satisfies `p`
-    match Classical.em (∀ n, ¬ p n) with
-    | .inl h =>
-      -- There doesn't exist: trivial
-      apply (Or.inl h)
-    | .inr h =>
-      -- There exists: we simply use `least_spec_aux` in combination with the property
-      -- of the epsilon operator
-      simp at *
-      let ⟨ n, hn ⟩ := h
-      apply Or.inr
-      have hl := least_spec_aux p n hn
-      have he := Classical.epsilon_spec hl
-      apply he
-
-  /-! # The fixed point definitions -/
-
-  def fix_fuel (n : Nat) (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : a) :
-    Result (b x) :=
-    match n with
-    | 0 => .div
-    | n + 1 =>
-      f (fix_fuel n f) x
-
-  @[simp] def fix_fuel_pred (f : ((x:a) → Result (b x)) → (x:a) → Result (b x))
-    (x : a) (n : Nat) :=
-    not (div? (fix_fuel n f x))
-
-  def fix_fuel_P (f : ((x:a) → Result (b x)) → (x:a) → Result (b x))
-    (x : a) (n : Nat) : Prop :=
-    fix_fuel_pred f x n
-
-  noncomputable
-  def fix (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : a) : Result (b x) :=
-    fix_fuel (least (fix_fuel_P f x)) f x
-
-  /-! # The validity property -/
-
-  -- Monotonicity relation over results
-  -- TODO: generalize (we should parameterize the definition by a relation over `a`)
-  def result_rel {a : Type u} (x1 x2 : Result a) : Prop :=
-    match x1 with
-    | div => True
-    | fail _ => x2 = x1
-    | ret _ => x2 = x1 -- TODO: generalize
-
-  -- Monotonicity relation over monadic arrows (i.e., Kleisli arrows)
-  def karrow_rel (k1 k2 : (x:a) → Result (b x)) : Prop :=
-    ∀ x, result_rel (k1 x) (k2 x)
-
-  -- Monotonicity property for function bodies
-  def is_mono (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) : Prop :=
-    ∀ {{k1 k2}}, karrow_rel k1 k2 → karrow_rel (f k1) (f k2)
-
-  -- "Continuity" property.
-  -- We need this, and this looks a lot like continuity. Also see this paper:
-  -- https://inria.hal.science/file/index/docid/216187/filename/tarski.pdf
-  -- We define our "continuity" criteria so that it gives us what we need to
-  -- prove the fixed-point equation, and we can also easily manipulate it.
-  def is_cont (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) : Prop :=
-    ∀ x, (Hdiv : ∀ n, fix_fuel (.succ n) f x = div) → f (fix f) x = div
-
-  /-! # The proof of the fixed-point equation -/
-  theorem fix_fuel_mono {f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}
-    (Hmono : is_mono f) :
-    ∀ {{n m}}, n ≤ m → karrow_rel (fix_fuel n f) (fix_fuel m f) := by
-    intros n
-    induction n
-    case zero => simp [karrow_rel, fix_fuel, result_rel]
-    case succ n1 Hi =>
-      intros m Hle x
-      simp [result_rel]
-      match m with
-      | 0 =>
-        exfalso
-        zify at *
-        linarith
-      | Nat.succ m1 =>
-        simp_arith at Hle
-        simp [fix_fuel]
-        have Hi := Hi Hle
-        have Hmono := Hmono Hi x
-        simp [result_rel] at Hmono
-        apply Hmono
-
-  @[simp] theorem neg_fix_fuel_P
-    {f : ((x:a) → Result (b x)) → (x:a) → Result (b x)} {x : a} {n : Nat} :
-    ¬ fix_fuel_P f x n ↔ (fix_fuel n f x = div) := by
-    simp [fix_fuel_P, div?]
-    cases fix_fuel n f x <;> simp  
-
-  theorem fix_fuel_fix_mono {f : ((x:a) → Result (b x)) → (x:a) → Result (b x)} (Hmono : is_mono f) :
-    ∀ n, karrow_rel (fix_fuel n f) (fix f) := by
-    intros n x
-    simp [result_rel]
-    have Hl := least_spec (fix_fuel_P f x)
-    simp at Hl
-    match Hl with
-    | .inl Hl => simp [*]
-    | .inr ⟨ Hl, Hn ⟩ =>
-      match Classical.em (fix_fuel n f x = div) with
-      | .inl Hd =>
-        simp [*]
-      | .inr Hd =>
-        have Hineq : least (fix_fuel_P f x) ≤ n := by
-          -- Proof by contradiction
-          cases Classical.em (least (fix_fuel_P f x) ≤ n) <;> simp [*]
-          simp at *
-          rename_i Hineq
-          have Hn := Hn n Hineq
-          contradiction
-        have Hfix : ¬ (fix f x = div) := by
-          simp [fix]
-          -- By property of the least upper bound
-          revert Hd Hl
-          -- TODO: there is no conversion to select the head of a function!
-          conv => lhs; apply congr_fun; apply congr_fun; apply congr_fun; simp [fix_fuel_P, div?]
-          cases fix_fuel (least (fix_fuel_P f x)) f x <;> simp
-        have Hmono := fix_fuel_mono Hmono Hineq x
-        simp [result_rel] at Hmono
-        simp [fix] at *
-        cases Heq: fix_fuel (least (fix_fuel_P f x)) f x <;>
-        cases Heq':fix_fuel n f x <;>
-        simp_all
-
-  theorem fix_fuel_P_least {f : ((x:a) → Result (b x)) → (x:a) → Result (b x)} (Hmono : is_mono f) :
-    ∀ {{x n}}, fix_fuel_P f x n → fix_fuel_P f x (least (fix_fuel_P f x)) := by
-    intros x n Hf
-    have Hfmono := fix_fuel_fix_mono Hmono n x
-    -- TODO: there is no conversion to select the head of a function!
-    conv => apply congr_fun; simp [fix_fuel_P]
-    simp [fix_fuel_P] at Hf
-    revert Hf Hfmono
-    simp [div?, result_rel, fix]
-    cases fix_fuel n f x <;> simp_all
-
-  -- Prove the fixed point equation in the case there exists some fuel for which
-  -- the execution terminates
-  theorem fix_fixed_eq_terminates (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (Hmono : is_mono f)
-    (x : a) (n : Nat) (He : fix_fuel_P f x n) :
-    fix f x = f (fix f) x := by
-    have Hl := fix_fuel_P_least Hmono He
-    -- TODO: better control of simplification
-    conv at Hl =>
-      apply congr_fun
-      simp [fix_fuel_P]
-    -- The least upper bound is > 0
-    have ⟨ n, Hsucc ⟩ : ∃ n, least (fix_fuel_P f x) = Nat.succ n := by
-      revert Hl
-      simp [div?]
-      cases least (fix_fuel_P f x) <;> simp [fix_fuel]
-    simp [Hsucc] at Hl
-    revert Hl
-    simp [*, div?, fix, fix_fuel]
-    -- Use the monotonicity
-    have Hfixmono := fix_fuel_fix_mono Hmono n
-    have Hvm := Hmono Hfixmono x
-    -- Use functional extensionality
-    simp [result_rel, fix] at Hvm
-    revert Hvm
-    split <;> simp [*] <;> intros <;> simp [*]
-
-  theorem fix_fixed_eq_forall {{f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
-    (Hmono : is_mono f) (Hcont : is_cont f) :
-    ∀ x, fix f x = f (fix f) x := by
-    intros x
-    -- Case disjunction: is there a fuel such that the execution successfully execute?
-    match Classical.em (∃ n, fix_fuel_P f x n) with
-    | .inr He =>
-      -- No fuel: the fixed point evaluates to `div`
-      --simp [fix] at *
-      simp at *
-      conv => lhs; simp [fix]
-      have Hel := He (Nat.succ (least (fix_fuel_P f x))); simp [*, fix_fuel] at *; clear Hel
-      -- Use the "continuity" of `f`
-      have He : ∀ n, fix_fuel (.succ n) f x = div := by intros; simp [*]
-      have Hcont := Hcont x He
-      simp [Hcont]
-    | .inl ⟨ n, He ⟩ => apply fix_fixed_eq_terminates f Hmono x n He
-
-  -- The final fixed point equation
-  theorem fix_fixed_eq {{f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
-    (Hmono : is_mono f) (Hcont : is_cont f) :
-    fix f = f (fix f) := by
-    have Heq := fix_fixed_eq_forall Hmono Hcont
-    have Heq1 : fix f = (λ x => fix f x) := by simp
-    rw [Heq1]
-    conv => lhs; ext; simp [Heq]
-
-  /-! # Making the proofs of validity manageable (and automatable) -/
-
-  -- Monotonicity property for expressions
-  def is_mono_p (e : ((x:a) → Result (b x)) → Result c) : Prop :=
-    ∀ {{k1 k2}}, karrow_rel k1 k2 → result_rel (e k1) (e k2)
-
-  theorem is_mono_p_same (x : Result c) :
-    @is_mono_p a b c (λ _ => x) := by
-    simp [is_mono_p, karrow_rel, result_rel]
-    split <;> simp
-
-  theorem is_mono_p_rec (x : a) :
-    @is_mono_p a b (b x) (λ f => f x) := by
-    simp_all [is_mono_p, karrow_rel, result_rel]
-
-  -- The important lemma about `is_mono_p`
-  theorem is_mono_p_bind
-    (g : ((x:a) → Result (b x)) → Result c)
-    (h : c → ((x:a) → Result (b x)) → Result d) :
-    is_mono_p g →
-    (∀ y, is_mono_p (h y)) →
-    @is_mono_p a b d (λ k => do let y ← g k; h y k) := by
-    intro hg hh
-    simp [is_mono_p]
-    intro fg fh Hrgh
-    simp [karrow_rel, result_rel]
-    have hg := hg Hrgh; simp [result_rel] at hg
-    cases heq0: g fg <;> simp_all
-    rename_i y _
-    have hh := hh y Hrgh; simp [result_rel] at hh
-    simp_all
-
-  -- Continuity property for expressions - note that we take the continuation
-  -- as parameter
-  def is_cont_p (k : ((x:a) → Result (b x)) → (x:a) → Result (b x))
-    (e : ((x:a) → Result (b x)) → Result c) : Prop :=
-    (Hc : ∀ n, e (fix_fuel n k) = .div) →
-    e (fix k) = .div
-
-  theorem is_cont_p_same (k : ((x:a) → Result (b x)) → (x:a) → Result (b x))
-    (x : Result c) :
-    is_cont_p k (λ _ => x) := by
-    simp [is_cont_p]
-
-  theorem is_cont_p_rec (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : a) :
-    is_cont_p f (λ f => f x) := by
-    simp_all [is_cont_p, fix]
-
-  -- The important lemma about `is_cont_p`
-  theorem is_cont_p_bind
-    (k : ((x:a) → Result (b x)) → (x:a) → Result (b x))
-    (Hkmono : is_mono k)
-    (g : ((x:a) → Result (b x)) → Result c)
-    (h : c → ((x:a) → Result (b x)) → Result d) :
-    is_mono_p g →
-    is_cont_p k g →
-    (∀ y, is_mono_p (h y)) →
-    (∀ y, is_cont_p k (h y)) →
-    is_cont_p k (λ k => do let y ← g k; h y k) := by
-    intro Hgmono Hgcont Hhmono Hhcont
-    simp [is_cont_p]
-    intro Hdiv
-    -- Case on `g (fix... k)`: is there an n s.t. it terminates?
-    cases Classical.em (∀ n, g (fix_fuel n k) = .div) <;> rename_i Hn
-    . -- Case 1: g diverges
-      have Hgcont := Hgcont Hn
-      simp_all
-    . -- Case 2: g doesn't diverge
-      simp at Hn
-      let ⟨ n, Hn ⟩ := Hn
-      have Hdivn := Hdiv n
-      have Hffmono := fix_fuel_fix_mono Hkmono n
-      have Hgeq := Hgmono Hffmono
-      simp [result_rel] at Hgeq
-      cases Heq: g (fix_fuel n k) <;> rename_i y <;> simp_all
-      -- Remains the .ret case
-      -- Use Hdiv to prove that: ∀ n, h y (fix_fuel n f) = div
-      -- We do this in two steps: first we prove it for m ≥ n
-      have Hhdiv: ∀ m, h y (fix_fuel m k) = .div := by
-        have Hhdiv : ∀ m, n ≤ m → h y (fix_fuel m k) = .div := by
-          -- We use the fact that `g (fix_fuel n f) = .div`, combined with Hdiv
-          intro m Hle
-          have Hdivm := Hdiv m
-          -- Monotonicity of g
-          have Hffmono := fix_fuel_mono Hkmono Hle
-          have Hgmono := Hgmono Hffmono
-          -- We need to clear Hdiv because otherwise simp_all rewrites Hdivm with Hdiv
-          clear Hdiv
-          simp_all [result_rel]
-        intro m
-        -- TODO: we shouldn't need the excluded middle here because it is decidable
-        cases Classical.em (n ≤ m) <;> rename_i Hl
-        . apply Hhdiv; assumption
-        . simp at Hl
-          -- Make a case disjunction on `h y (fix_fuel m k)`: if it is not equal
-          -- to div, use the monotonicity of `h y`
-          have Hle : m ≤ n := by linarith
-          have Hffmono := fix_fuel_mono Hkmono Hle
-          have Hmono := Hhmono y Hffmono
-          simp [result_rel] at Hmono
-          cases Heq: h y (fix_fuel m k) <;> simp_all
-      -- We can now use the continuity hypothesis for h
-      apply Hhcont; assumption
-
-  -- The validity property for an expression
-  def is_valid_p (k : ((x:a) → Result (b x)) → (x:a) → Result (b x))
-    (e : ((x:a) → Result (b x)) → Result c) : Prop :=
-    is_mono_p e ∧
-    (is_mono k → is_cont_p k e)
-
-  @[simp] theorem is_valid_p_same
-    (k : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : Result c) :
-    is_valid_p k (λ _ => x) := by
-    simp [is_valid_p, is_mono_p_same, is_cont_p_same]
-
-  @[simp] theorem is_valid_p_rec
-    (k : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : a) :
-    is_valid_p k (λ k => k x) := by
-    simp_all [is_valid_p, is_mono_p_rec, is_cont_p_rec]
-
-  -- Lean is good at unification: we can write a very general version
-  -- (in particular, it will manage to figure out `g` and `h` when we
-  -- apply the lemma)
-  theorem is_valid_p_bind
-    {{k : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
-    {{g : ((x:a) → Result (b x)) → Result c}}
-    {{h : c → ((x:a) → Result (b x)) → Result d}}
-    (Hgvalid : is_valid_p k g)
-    (Hhvalid : ∀ y, is_valid_p k (h y)) :
-    is_valid_p k (λ k => do let y ← g k; h y k) := by
-    let ⟨ Hgmono, Hgcont ⟩ := Hgvalid
-    simp [is_valid_p, forall_and] at Hhvalid
-    have ⟨ Hhmono, Hhcont ⟩ := Hhvalid
-    simp [← imp_forall_iff] at Hhcont
-    simp [is_valid_p]; constructor
-    . -- Monotonicity
-      apply is_mono_p_bind <;> assumption
-    . -- Continuity
-      intro Hkmono
-      have Hgcont := Hgcont Hkmono
-      have Hhcont := Hhcont Hkmono
-      apply is_cont_p_bind <;> assumption
-
-  def is_valid (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) : Prop :=
-    ∀ k x, is_valid_p k (λ k => f k x)
-
-  theorem is_valid_p_imp_is_valid {{f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
-    (Hvalid : is_valid f) :
-    is_mono f ∧ is_cont f := by
-    have Hmono : is_mono f := by
-      intro f h Hr x
-      have Hmono := Hvalid (λ _ _ => .div) x
-      have Hmono := Hmono.left
-      apply Hmono; assumption
-    have Hcont : is_cont f := by
-      intro x Hdiv
-      have Hcont := (Hvalid f x).right Hmono
-      simp [is_cont_p] at Hcont
-      apply Hcont
-      intro n
-      have Hdiv := Hdiv n
-      simp [fix_fuel] at Hdiv
-      simp [*]
-    simp [*]
-
-  theorem is_valid_fix_fixed_eq {{f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
-    (Hvalid : is_valid f) :
-    fix f = f (fix f) := by
-    have ⟨ Hmono, Hcont ⟩ := is_valid_p_imp_is_valid Hvalid
-    exact fix_fixed_eq Hmono Hcont
-
-end Fix
-
-namespace FixI
-  /- Indexed fixed-point: definitions with indexed types, convenient to use for mutually
-     recursive definitions. We simply port the definitions and proofs from Fix to a more
-     specific case.
-   -/
-  open Primitives Fix
-
-  -- The index type
-  variable {id : Type}
-
-  -- The input/output types
-  variable {a b : id → Type}
-
-  -- Monotonicity relation over monadic arrows (i.e., Kleisli arrows)
-  def karrow_rel (k1 k2 : (i:id) → a i → Result (b i)) : Prop :=
-    ∀ i x, result_rel (k1 i x) (k2 i x)
-
-  def kk_to_gen (k : (i:id) → a i → Result (b i)) :
-    (x: (i:id) × a i) → Result (b x.fst) :=
-    λ ⟨ i, x ⟩ => k i x
-
-  def kk_of_gen (k : (x: (i:id) × a i) → Result (b x.fst)) :
-    (i:id) → a i → Result (b i) :=
-    λ i x => k ⟨ i, x ⟩
-
-  def k_to_gen (k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) :
-    ((x: (i:id) × a i) → Result (b x.fst)) → (x: (i:id) × a i) → Result (b x.fst) :=
-    λ kk => kk_to_gen (k (kk_of_gen kk))
-
-  def k_of_gen (k : ((x: (i:id) × a i) → Result (b x.fst)) → (x: (i:id) × a i) → Result (b x.fst)) :
-    ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i) :=
-    λ kk => kk_of_gen (k (kk_to_gen kk))
-
-  def e_to_gen (e : ((i:id) → a i → Result (b i)) → Result c) :
-    ((x: (i:id) × a i) → Result (b x.fst)) → Result c :=
-    λ k => e (kk_of_gen k)
-
-  def is_valid_p (k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i))
-    (e : ((i:id) → a i → Result (b i)) → Result c) : Prop :=
-    Fix.is_valid_p (k_to_gen k) (e_to_gen e)
-
-  def is_valid (f : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) : Prop :=
-    ∀ k i x, is_valid_p k (λ k => f k i x)
-
-  noncomputable def fix
-    (f : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) :
-    (i:id) → a i → Result (b i) :=
-    kk_of_gen (Fix.fix (k_to_gen f))
-
-  theorem is_valid_fix_fixed_eq
-    {{f : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)}}
-    (Hvalid : is_valid f) :
-    fix f = f (fix f) := by
-    have Hvalid' : Fix.is_valid (k_to_gen f) := by
-      intro k x
-      simp only [is_valid, is_valid_p] at Hvalid
-      let ⟨ i, x ⟩ := x
-      have Hvalid := Hvalid (k_of_gen k) i x
-      simp only [k_to_gen, k_of_gen, kk_to_gen, kk_of_gen] at Hvalid
-      refine Hvalid
-    have Heq := Fix.is_valid_fix_fixed_eq Hvalid'
-    simp [fix]
-    conv => lhs; rw [Heq]
-
-  /- Some utilities to define the mutually recursive functions -/
-
-  -- TODO: use more
-  @[simp] def kk_ty (id : Type) (a b : id → Type) := (i:id) → a i → Result (b i)
-  @[simp] def k_ty (id : Type) (a b : id → Type) := kk_ty id a b → kk_ty id a b
-
-  -- Initially, we had left out the parameters id, a and b.
-  -- However, by parameterizing Funs with those parameters, we can state
-  -- and prove lemmas like Funs.is_valid_p_is_valid_p
-  inductive Funs (id : Type) (a b : id → Type) :
-    List (Type u) → List (Type u) → Type (u + 1) :=
-    | Nil : Funs id a b [] []
-    | Cons {ity oty : Type u} {itys otys : List (Type u)}
-           (f : kk_ty id a b → ity → Result oty) (tl : Funs id a b itys otys) :
-           Funs id a b (ity :: itys) (oty :: otys)
-
-  theorem Funs.length_eq {itys otys : List (Type)} (fl : Funs id a b itys otys) :
-    otys.length = itys.length :=
-    match fl with
-    | .Nil => by simp
-    | .Cons f tl =>
-      have h:= Funs.length_eq tl
-      by simp [h]
-
-  def fin_cast {n m : Nat} (h : m = n) (i : Fin n) : Fin m :=
-    ⟨ i.val, by have h1:= i.isLt; simp_all ⟩
-
-  @[simp] def Funs.cast_fin {itys otys : List (Type)}
-    (fl : Funs id a b itys otys) (i : Fin itys.length) : Fin otys.length :=
-    fin_cast (fl.length_eq) i
-
-  def get_fun {itys otys : List (Type)} (fl : Funs id a b itys otys) :
-    (i : Fin itys.length) → kk_ty id a b → itys.get i → Result (otys.get (fl.cast_fin i)) :=
-    match fl with
-    | .Nil => λ i => by have h:= i.isLt; simp at h
-    | @Funs.Cons id a b ity oty itys1 otys1 f tl =>
-      λ i =>
-      if h: i.val = 0 then
-        Eq.mp (by cases i; simp_all [List.get]) f
-      else
-        let j := i.val - 1
-        have Hj: j < itys1.length := by
-          have Hi := i.isLt
-          simp at Hi
-          revert Hi
-          cases Heq: i.val <;> simp_all
-          simp_arith
-        let j: Fin itys1.length := ⟨ j, Hj ⟩
-        Eq.mp
-          (by
-            cases Heq: i; rename_i val isLt;
-            cases Heq': j; rename_i val' isLt;
-            cases val <;> simp_all [List.get, fin_cast])
-          (get_fun tl j)
-
-  -- TODO: move
-  theorem add_one_le_iff_le_ne (n m : Nat) (h1 : m ≤ n) (h2 : m ≠ n) : m + 1 ≤ n := by
-    -- Damn, those proofs on natural numbers are hard - I wish Omega was in mathlib4...
-    simp [Nat.add_one_le_iff]
-    simp [Nat.lt_iff_le_and_ne]
-    simp_all
-
-  def for_all_fin_aux {n : Nat} (f : Fin n → Prop) (m : Nat) (h : m ≤ n) : Prop :=
-    if heq: m = n then True
-    else
-      f ⟨ m, by simp_all [Nat.lt_iff_le_and_ne] ⟩ ∧
-      for_all_fin_aux f (m + 1) (by simp_all [add_one_le_iff_le_ne])
-  termination_by for_all_fin_aux n _ m h => n - m
-  decreasing_by
-    simp_wf
-    apply Nat.sub_add_lt_sub <;> simp
-    simp_all [add_one_le_iff_le_ne]
-
-  def for_all_fin {n : Nat} (f : Fin n → Prop) := for_all_fin_aux f 0 (by simp)
-
-  theorem for_all_fin_aux_imp_forall {n : Nat} (f : Fin n → Prop) (m : Nat) :
-    (h : m ≤ n) →
-    for_all_fin_aux f m h → ∀ i, m ≤ i.val → f i
-    := by
-    generalize h: (n - m) = k
-    revert m
-    induction k -- TODO: induction h rather?
-    case zero =>
-      simp_all
-      intro m h1 h2
-      have h: n = m := by
-        linarith
-      unfold for_all_fin_aux; simp_all
-      simp_all
-      -- There is no i s.t. m ≤ i
-      intro i h3; cases i; simp_all
-      linarith
-    case succ k hi =>
-      simp_all
-      intro m hk hmn
-      intro hf i hmi
-      have hne: m ≠ n := by
-        have hineq := Nat.lt_of_sub_eq_succ hk
-        linarith
-      -- m = i?
-      if heq: m = i then
-        -- Yes: simply use the `for_all_fin_aux` hyp
-        unfold for_all_fin_aux at hf
-        simp_all
-        tauto
-      else
-        -- No: use the induction hypothesis
-        have hlt: m < i := by simp_all [Nat.lt_iff_le_and_ne]
-        have hineq: m + 1 ≤ n := by
-          have hineq := Nat.lt_of_sub_eq_succ hk
-          simp [*, Nat.add_one_le_iff]
-        have heq1: n - (m + 1) = k := by
-          -- TODO: very annoying arithmetic proof
-          simp [Nat.sub_eq_iff_eq_add hineq]
-          have hineq1: m ≤ n := by linarith
-          simp [Nat.sub_eq_iff_eq_add hineq1] at hk
-          simp_arith [hk]
-        have hi := hi (m + 1) heq1 hineq
-        apply hi <;> simp_all
-        . unfold for_all_fin_aux at hf
-          simp_all
-        . simp_all [add_one_le_iff_le_ne]
-
-  -- TODO: this is not necessary anymore
-  theorem for_all_fin_imp_forall (n : Nat) (f : Fin n → Prop) :
-    for_all_fin f → ∀ i, f i
-    := by
-    intro Hf i
-    apply for_all_fin_aux_imp_forall <;> try assumption
-    simp
-
-  /- Automating the proofs -/
-  @[simp] theorem is_valid_p_same
-    (k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) (x : Result c) :
-    is_valid_p k (λ _ => x) := by
-    simp [is_valid_p, k_to_gen, e_to_gen]
-
-  @[simp] theorem is_valid_p_rec
-     (k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) (i : id) (x : a i) :
-    is_valid_p k (λ k => k i x) := by
-    simp [is_valid_p, k_to_gen, e_to_gen, kk_to_gen, kk_of_gen]
-
-  theorem is_valid_p_bind
-    {{k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)}}
-    {{g : ((i:id) → a i → Result (b i)) → Result c}}
-    {{h : c → ((i:id) → a i → Result (b i)) → Result d}}
-    (Hgvalid : is_valid_p k g)
-    (Hhvalid : ∀ y, is_valid_p k (h y)) :
-    is_valid_p k (λ k => do let y ← g k; h y k) := by
-    apply Fix.is_valid_p_bind
-    . apply Hgvalid
-    . apply Hhvalid
-
-  def Funs.is_valid_p
-    (k : k_ty id a b)
-    (fl : Funs id a b itys otys) :
-    Prop :=
-    match fl with
-    | .Nil => True
-    | .Cons f fl => (∀ x, FixI.is_valid_p k (λ k => f k x)) ∧ fl.is_valid_p k
-
-  def Funs.is_valid_p_is_valid_p_aux
-    {k : k_ty id a b}
-    {itys otys : List Type}
-    (Heq : List.length otys = List.length itys)
-    (fl : Funs id a b itys otys) (Hvalid : is_valid_p k fl) :
-    ∀ (i : Fin itys.length) (x : itys.get i), FixI.is_valid_p k (fun k => get_fun fl i k x) := by
-    -- Prepare the induction
-    have ⟨ n, Hn ⟩ : { n : Nat // itys.length = n } := ⟨ itys.length, by rfl ⟩
-    revert itys otys Heq fl Hvalid
-    induction n
-    --
-    case zero =>
-      intro itys otys Heq fl Hvalid Hlen;
-      have Heq: itys = [] := by cases itys <;> simp_all
-      have Heq: otys = [] := by cases otys <;> simp_all
-      intro i x
-      simp_all
-      have Hi := i.isLt
-      simp_all
-    case succ n Hn =>
-      intro itys otys Heq fl Hvalid Hlen i x;
-      cases fl <;> simp at Hlen i x Heq Hvalid
-      rename_i ity oty itys otys f fl
-      have ⟨ Hvf, Hvalid ⟩ := Hvalid
-      have Hvf1: is_valid_p k fl := by
-        simp [Hvalid, Funs.is_valid_p]
-      have Hn := @Hn itys otys (by simp[*]) fl Hvf1 (by simp [*])
-      -- Case disjunction on i
-      match i with
-      | ⟨ 0, _ ⟩ =>
-        simp at x
-        simp [get_fun]
-        apply (Hvf x)
-      | ⟨ .succ j, HiLt ⟩ =>
-        simp_arith at HiLt
-        simp at x
-        let j : Fin (List.length itys) := ⟨ j, by simp_arith [HiLt] ⟩
-        have Hn := Hn j x
-        apply Hn
-
-  def Funs.is_valid_p_is_valid_p
-    (itys otys : List (Type)) (Heq: otys.length = itys.length := by decide)
-    (k : k_ty (Fin (List.length itys)) (List.get itys) fun i => List.get otys (fin_cast Heq i))
-    (fl : Funs (Fin itys.length) itys.get (λ i => otys.get (fin_cast Heq i)) itys otys) :
-    fl.is_valid_p k  →
-    ∀ (i : Fin itys.length) (x : itys.get i), FixI.is_valid_p k (fun k => get_fun fl i k x)
-    := by
-    intro Hvalid
-    apply is_valid_p_is_valid_p_aux <;> simp [*]
-
-end FixI
-
-namespace Ex1
-  /- An example of use of the fixed-point -/
-  open Primitives Fix
-
-  variable {a : Type} (k : (List a × Int) → Result a)
-
-  def list_nth_body (x : (List a × Int)) : Result a :=
-    let (ls, i) := x
-    match ls with
-    | [] => .fail .panic
-    | hd :: tl =>
-      if i = 0 then .ret hd
-      else k (tl, i - 1)
-
-  theorem list_nth_body_is_valid: ∀ k x, is_valid_p k (λ k => @list_nth_body a k x) := by
-    intro k x
-    simp [list_nth_body]
-    split <;> simp
-    split <;> simp
-
-  noncomputable
-  def list_nth (ls : List a) (i : Int) : Result a := fix list_nth_body (ls, i)
-
-  -- The unfolding equation - diverges if `i < 0`
-  theorem list_nth_eq (ls : List a) (i : Int) :
-    list_nth ls i =
-      match ls with
-      | [] => .fail .panic
-      | hd :: tl =>
-        if i = 0 then .ret hd
-        else list_nth tl (i - 1)
-    := by
-    have Heq := is_valid_fix_fixed_eq (@list_nth_body_is_valid a)
-    simp [list_nth]
-    conv => lhs; rw [Heq]
-
-end Ex1
-
-namespace Ex2
-  /- Same as Ex1, but we make the body of nth non tail-rec (this is mostly
-     to see what happens when there are let-bindings) -/
-  open Primitives Fix
-
-  variable {a : Type} (k : (List a × Int) → Result a)
-
-  def list_nth_body (x : (List a × Int)) : Result a :=
-    let (ls, i) := x
-    match ls with
-    | [] => .fail .panic
-    | hd :: tl =>
-      if i = 0 then .ret hd
-      else
-        do
-          let y ← k (tl, i - 1)
-          .ret y
-
-  theorem list_nth_body_is_valid: ∀ k x, is_valid_p k (λ k => @list_nth_body a k x) := by
-    intro k x
-    simp [list_nth_body]
-    split <;> simp
-    split <;> simp
-    apply is_valid_p_bind <;> intros <;> simp_all
-
-  noncomputable
-  def list_nth (ls : List a) (i : Int) : Result a := fix list_nth_body (ls, i)
-
-  -- The unfolding equation - diverges if `i < 0`
-  theorem list_nth_eq (ls : List a) (i : Int) :
-    (list_nth ls i =
-       match ls with
-       | [] => .fail .panic
-       | hd :: tl =>
-         if i = 0 then .ret hd
-         else
-           do
-             let y ← list_nth tl (i - 1)
-             .ret y)
-    := by
-    have Heq := is_valid_fix_fixed_eq (@list_nth_body_is_valid a)
-    simp [list_nth]
-    conv => lhs; rw [Heq]
-
-end Ex2
-
-namespace Ex3
-  /- Mutually recursive functions - first encoding (see Ex4 for a better encoding) -/
-  open Primitives Fix
-
-  /- Because we have mutually recursive functions, we use a sum for the inputs
-     and the output types:
-     - inputs: the sum allows to select the function to call in the recursive
-       calls (and the functions may not have the same input types)
-     - outputs: this case is degenerate because `even` and `odd` have the same
-       return type `Bool`, but generally speaking we need a sum type because
-       the functions in the mutually recursive group may have different
-       return types.
-   -/
-  variable (k : (Int ⊕ Int) → Result (Bool ⊕ Bool))
-
-  def is_even_is_odd_body (x : (Int ⊕ Int)) : Result (Bool ⊕ Bool) :=
-    match x with
-    | .inl i =>
-      -- Body of `is_even`
-      if i = 0
-      then .ret (.inl true) -- We use .inl because this is `is_even`
-      else
-        do
-          let b ←
-            do
-              -- Call `odd`: we need to wrap the input value in `.inr`, then
-              -- extract the output value
-              let r ← k (.inr (i- 1))
-              match r with
-              | .inl _ => .fail .panic -- Invalid output
-              | .inr b => .ret b
-          -- Wrap the return value
-          .ret (.inl b)
-    | .inr i =>
-      -- Body of `is_odd`
-      if i = 0
-      then .ret (.inr false) -- We use .inr because this is `is_odd`
-      else
-        do
-          let b ←
-            do
-              -- Call `is_even`: we need to wrap the input value in .inr, then
-              -- extract the output value
-              let r ← k (.inl (i- 1))
-              match r with
-              | .inl b => .ret b
-              | .inr _ => .fail .panic -- Invalid output
-          -- Wrap the return value
-          .ret (.inr b)
-
-  theorem is_even_is_odd_body_is_valid:
-    ∀ k x, is_valid_p k (λ k => is_even_is_odd_body k x) := by
-    intro k x
-    simp [is_even_is_odd_body]
-    split <;> simp <;> split <;> simp
-    apply is_valid_p_bind; simp
-    intros; split <;> simp
-    apply is_valid_p_bind; simp
-    intros; split <;> simp
-
-  noncomputable
-  def is_even (i : Int): Result Bool :=
-    do
-      let r ← fix is_even_is_odd_body (.inl i)
-      match r with
-      | .inl b => .ret b
-      | .inr _ => .fail .panic
-
-  noncomputable
-  def is_odd (i : Int): Result Bool :=
-    do
-      let r ← fix is_even_is_odd_body (.inr i)
-      match r with
-      | .inl _ => .fail .panic
-      | .inr b => .ret b
-
-  -- The unfolding equation for `is_even` - diverges if `i < 0`
-  theorem is_even_eq (i : Int) :
-    is_even i = (if i = 0 then .ret true else is_odd (i - 1))
-    := by
-    have Heq := is_valid_fix_fixed_eq is_even_is_odd_body_is_valid
-    simp [is_even, is_odd]
-    conv => lhs; rw [Heq]; simp; rw [is_even_is_odd_body]; simp
-    -- Very annoying: we need to swap the matches
-    -- Doing this with rewriting lemmas is hard generally speaking
-    -- (especially as we may have to generate lemmas for user-defined
-    -- inductives on the fly).
-    -- The simplest is to repeatedly split then simplify (we identify
-    -- the outer match or monadic let-binding, and split on its scrutinee)
-    split <;> simp
-    cases H0 : fix is_even_is_odd_body (Sum.inr (i - 1)) <;> simp
-    rename_i v
-    split <;> simp
-
-  -- The unfolding equation for `is_odd` - diverges if `i < 0`
-  theorem is_odd_eq (i : Int) :
-    is_odd i = (if i = 0 then .ret false else is_even (i - 1))
-    := by
-    have Heq := is_valid_fix_fixed_eq is_even_is_odd_body_is_valid
-    simp [is_even, is_odd]
-    conv => lhs; rw [Heq]; simp; rw [is_even_is_odd_body]; simp
-    -- Same remark as for `even`
-    split <;> simp
-    cases H0 : fix is_even_is_odd_body (Sum.inl (i - 1)) <;> simp
-    rename_i v
-    split <;> simp
-
-end Ex3
-
-namespace Ex4
-  /- Mutually recursive functions - 2nd encoding -/
-  open Primitives FixI
-
-  attribute [local simp] List.get
-
-  /- We make the input type and output types dependent on a parameter -/
-  @[simp] def input_ty (i : Fin 2) : Type :=
-    [Int, Int].get i
-
-  @[simp] def output_ty (i : Fin 2) : Type :=
-    [Bool, Bool].get i
-
-  /- The continuation -/
-  variable (k : (i : Fin 2) → input_ty i → Result (output_ty i))
-
-  /- The bodies are more natural -/
-  def is_even_body (k : (i : Fin 2) → input_ty i → Result (output_ty i)) (i : Int) : Result Bool :=
-    if i = 0
-      then .ret true
-    else do
-      let b ← k 1 (i - 1)
-      .ret b
-
-  def is_odd_body (i : Int) : Result Bool :=
-    if i = 0
-      then .ret false
-    else do
-      let b ← k 0 (i - 1)
-      .ret b
-
-  @[simp] def bodies :
-    Funs (Fin 2) input_ty output_ty [Int, Int] [Bool, Bool] :=
-    Funs.Cons (is_even_body) (Funs.Cons (is_odd_body) Funs.Nil)
-
-  def body (k : (i : Fin 2) → input_ty i → Result (output_ty i)) (i: Fin 2) :
-    input_ty i → Result (output_ty i) := get_fun bodies i k
-
-  theorem body_is_valid : is_valid body := by
-    -- Split the proof into proofs of validity of the individual bodies
-    rw [is_valid]
-    simp only [body]
-    intro k
-    apply (Funs.is_valid_p_is_valid_p [Int, Int] [Bool, Bool])
-    simp [Funs.is_valid_p]
-    (repeat (apply And.intro)) <;> intro x <;> simp at x <;>
-    simp only [is_even_body, is_odd_body]
-    -- Prove the validity of the individual bodies
-    . split <;> simp
-      apply is_valid_p_bind <;> simp
-    . split <;> simp
-      apply is_valid_p_bind <;> simp
-
-  theorem body_fix_eq : fix body = body (fix body) :=
-    is_valid_fix_fixed_eq body_is_valid
-
-  noncomputable def is_even (i : Int) : Result Bool := fix body 0 i
-  noncomputable def is_odd (i : Int) : Result Bool := fix body 1 i
-
-  theorem is_even_eq (i : Int) : is_even i =
-    (if i = 0
-       then .ret true
-     else do
-       let b ← is_odd (i - 1)
-       .ret b) := by
-    simp [is_even, is_odd];
-    conv => lhs; rw [body_fix_eq]
-
-  theorem is_odd_eq (i : Int) : is_odd i =
-    (if i = 0
-       then .ret false
-     else do
-       let b ← is_even (i - 1)
-       .ret b) := by
-    simp [is_even, is_odd];
-    conv => lhs; rw [body_fix_eq]
-
-end Ex4
-
-namespace Ex5
-  /- Higher-order example -/
-  open Primitives Fix
-
-  variable {a b : Type}
-
-  /- An auxiliary function, which doesn't require the fixed-point -/
-  def map (f : a → Result b) (ls : List a) : Result (List b) :=
-    match ls with
-    | [] => .ret []
-    | hd :: tl =>
-      do
-        let hd ← f hd
-        let tl ← map f tl
-        .ret (hd :: tl)
-
-  /- The validity theorem for `map`, generic in `f` -/
-  theorem map_is_valid
-    {{f : (a → Result b) → a → Result c}}
-    (Hfvalid : ∀ k x, is_valid_p k (λ k => f k x))
-    (k : (a → Result b) → a → Result b)
-    (ls : List a) :
-    is_valid_p k (λ k => map (f k) ls) := by
-    induction ls <;> simp [map]
-    apply is_valid_p_bind <;> simp_all
-    intros
-    apply is_valid_p_bind <;> simp_all
-
-  /- An example which uses map -/
-  inductive Tree (a : Type) :=
-  | leaf (x : a)
-  | node (tl : List (Tree a))
-
-  def id_body (k : Tree a → Result (Tree a)) (t : Tree a) : Result (Tree a) :=
-    match t with
-    | .leaf x => .ret (.leaf x)
-    | .node tl =>
-      do
-        let tl ← map k tl
-        .ret (.node tl)
-
-  theorem id_body_is_valid :
-    ∀ k x, is_valid_p k (λ k => @id_body a k x) := by
-    intro k x
-    simp only [id_body]
-    split <;> simp
-    apply is_valid_p_bind <;> simp [*]
-    -- We have to show that `map k tl` is valid
-    apply map_is_valid;
-    -- Remark: if we don't do the intro, then the last step is expensive:
-    -- "typeclass inference of Nonempty took 119ms"
-    intro k x
-    simp only [is_valid_p_same, is_valid_p_rec]
-
-  noncomputable def id (t : Tree a) := fix id_body t
-
-  -- The unfolding equation
-  theorem id_eq (t : Tree a) :
-    (id t =
-       match t with
-       | .leaf x => .ret (.leaf x)
-       | .node tl =>
-       do
-         let tl ← map id tl
-         .ret (.node tl))
-    := by
-    have Heq := is_valid_fix_fixed_eq (@id_body_is_valid a)
-    simp [id]
-    conv => lhs; rw [Heq]; simp; rw [id_body]
-
-end Ex5
-
-end Diverge
+import Base.Diverge.Base
+import Base.Diverge.Elab
diff --git a/backends/lean/Base/Diverge/Base.lean b/backends/lean/Base/Diverge/Base.lean
new file mode 100644
index 00000000..0f92e682
--- /dev/null
+++ b/backends/lean/Base/Diverge/Base.lean
@@ -0,0 +1,1105 @@
+import Lean
+import Lean.Meta.Tactic.Simp
+import Init.Data.List.Basic
+import Mathlib.Tactic.RunCmd
+import Mathlib.Tactic.Linarith
+
+/-
+TODO:
+- we want an easier to use cases:
+  - keeps in the goal an equation of the shape: `t = case`
+  - if called on Prop terms, uses Classical.em
+  Actually, the cases from mathlib seems already quite powerful
+  (https://leanprover-community.github.io/mathlib_docs/tactics.html#cases)
+  For instance: cases h : e
+  Also: cases_matching
+- better split tactic
+- we need conversions to operate on the head of applications.
+  Actually, something like this works:
+  ```
+  conv at Hl =>
+    apply congr_fun
+    simp [fix_fuel_P]
+  ```
+  Maybe we need a rpt ... ; focus?
+- simplifier/rewriter have a strange behavior sometimes
+-/
+
+
+/- TODO: this is very useful, but is there more? -/
+set_option profiler true
+set_option profiler.threshold 100
+
+namespace Diverge
+
+namespace Primitives
+/-! # Copy-pasting from Primitives to make the file self-contained -/
+
+inductive Error where
+   | assertionFailure: Error
+   | integerOverflow: Error
+   | divisionByZero: Error
+   | arrayOutOfBounds: Error
+   | maximumSizeExceeded: Error
+   | panic: Error
+deriving Repr, BEq
+
+open Error
+
+inductive Result (α : Type u) where
+  | ret (v: α): Result α
+  | fail (e: Error): Result α
+  | div
+deriving Repr, BEq
+
+open Result
+
+def bind (x: Result α) (f: α -> Result β) : Result β :=
+  match x with
+  | ret v  => f v 
+  | fail v => fail v
+  | div => div
+
+@[simp] theorem bind_ret (x : α) (f : α → Result β) : bind (.ret x) f = f x := by simp [bind]
+@[simp] theorem bind_fail (x : Error) (f : α → Result β) : bind (.fail x) f = .fail x := by simp [bind]
+@[simp] theorem bind_div (f : α → Result β) : bind .div f = .div := by simp [bind]
+
+-- Allows using Result in do-blocks
+instance : Bind Result where
+  bind := bind
+
+-- Allows using return x in do-blocks
+instance : Pure Result where
+  pure := fun x => ret x
+
+@[simp] theorem bind_tc_ret (x : α) (f : α → Result β) :
+  (do let y ← .ret x; f y) = f x := by simp [Bind.bind, bind]
+
+@[simp] theorem bind_tc_fail (x : Error) (f : α → Result β) :
+  (do let y ← fail x; f y) = fail x := by simp [Bind.bind, bind]
+
+@[simp] theorem bind_tc_div (f : α → Result β) :
+  (do let y ← div; f y) = div := by simp [Bind.bind, bind]
+
+def div? {α: Type} (r: Result α): Bool :=
+  match r with
+  | div => true
+  | ret _ | fail _ => false
+
+end Primitives
+
+namespace Fix
+
+  open Primitives
+  open Result
+
+  variable {a : Type} {b : a → Type}
+  variable {c d : Type}
+
+  /-! # The least fixed point definition and its properties -/
+
+  def least_p (p : Nat → Prop) (n : Nat) : Prop := p n ∧ (∀ m, m < n → ¬ p m)
+  noncomputable def least (p : Nat → Prop) : Nat :=
+    Classical.epsilon (least_p p)
+
+  -- Auxiliary theorem for [least_spec]: if there exists an `n` satisfying `p`,
+  -- there there exists a least `m` satisfying `p`.
+  theorem least_spec_aux (p : Nat → Prop) : ∀ (n : Nat), (hn : p n) → ∃ m, least_p p m := by
+    apply Nat.strongRec'
+    intros n hi hn
+    -- Case disjunction on: is n the smallest n satisfying p?
+    match Classical.em (∀ m, m < n → ¬ p m) with
+    | .inl hlt =>
+      -- Yes: trivial
+      exists n
+    | .inr hlt =>
+      simp at *
+      let ⟨ m, ⟨ hmlt, hm ⟩ ⟩ := hlt
+      have hi := hi m hmlt hm
+      apply hi
+
+  -- The specification of [least]: either `p` is never satisfied, or it is satisfied
+  -- by `least p` and no `n < least p` satisfies `p`.
+  theorem least_spec (p : Nat → Prop) : (∀ n, ¬ p n) ∨ (p (least p) ∧ ∀ n, n < least p → ¬ p n) := by
+    -- Case disjunction on the existence of an `n` which satisfies `p`
+    match Classical.em (∀ n, ¬ p n) with
+    | .inl h =>
+      -- There doesn't exist: trivial
+      apply (Or.inl h)
+    | .inr h =>
+      -- There exists: we simply use `least_spec_aux` in combination with the property
+      -- of the epsilon operator
+      simp at *
+      let ⟨ n, hn ⟩ := h
+      apply Or.inr
+      have hl := least_spec_aux p n hn
+      have he := Classical.epsilon_spec hl
+      apply he
+
+  /-! # The fixed point definitions -/
+
+  def fix_fuel (n : Nat) (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : a) :
+    Result (b x) :=
+    match n with
+    | 0 => .div
+    | n + 1 =>
+      f (fix_fuel n f) x
+
+  @[simp] def fix_fuel_pred (f : ((x:a) → Result (b x)) → (x:a) → Result (b x))
+    (x : a) (n : Nat) :=
+    not (div? (fix_fuel n f x))
+
+  def fix_fuel_P (f : ((x:a) → Result (b x)) → (x:a) → Result (b x))
+    (x : a) (n : Nat) : Prop :=
+    fix_fuel_pred f x n
+
+  noncomputable
+  def fix (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : a) : Result (b x) :=
+    fix_fuel (least (fix_fuel_P f x)) f x
+
+  /-! # The validity property -/
+
+  -- Monotonicity relation over results
+  -- TODO: generalize (we should parameterize the definition by a relation over `a`)
+  def result_rel {a : Type u} (x1 x2 : Result a) : Prop :=
+    match x1 with
+    | div => True
+    | fail _ => x2 = x1
+    | ret _ => x2 = x1 -- TODO: generalize
+
+  -- Monotonicity relation over monadic arrows (i.e., Kleisli arrows)
+  def karrow_rel (k1 k2 : (x:a) → Result (b x)) : Prop :=
+    ∀ x, result_rel (k1 x) (k2 x)
+
+  -- Monotonicity property for function bodies
+  def is_mono (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) : Prop :=
+    ∀ {{k1 k2}}, karrow_rel k1 k2 → karrow_rel (f k1) (f k2)
+
+  -- "Continuity" property.
+  -- We need this, and this looks a lot like continuity. Also see this paper:
+  -- https://inria.hal.science/file/index/docid/216187/filename/tarski.pdf
+  -- We define our "continuity" criteria so that it gives us what we need to
+  -- prove the fixed-point equation, and we can also easily manipulate it.
+  def is_cont (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) : Prop :=
+    ∀ x, (Hdiv : ∀ n, fix_fuel (.succ n) f x = div) → f (fix f) x = div
+
+  /-! # The proof of the fixed-point equation -/
+  theorem fix_fuel_mono {f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}
+    (Hmono : is_mono f) :
+    ∀ {{n m}}, n ≤ m → karrow_rel (fix_fuel n f) (fix_fuel m f) := by
+    intros n
+    induction n
+    case zero => simp [karrow_rel, fix_fuel, result_rel]
+    case succ n1 Hi =>
+      intros m Hle x
+      simp [result_rel]
+      match m with
+      | 0 =>
+        exfalso
+        zify at *
+        linarith
+      | Nat.succ m1 =>
+        simp_arith at Hle
+        simp [fix_fuel]
+        have Hi := Hi Hle
+        have Hmono := Hmono Hi x
+        simp [result_rel] at Hmono
+        apply Hmono
+
+  @[simp] theorem neg_fix_fuel_P
+    {f : ((x:a) → Result (b x)) → (x:a) → Result (b x)} {x : a} {n : Nat} :
+    ¬ fix_fuel_P f x n ↔ (fix_fuel n f x = div) := by
+    simp [fix_fuel_P, div?]
+    cases fix_fuel n f x <;> simp  
+
+  theorem fix_fuel_fix_mono {f : ((x:a) → Result (b x)) → (x:a) → Result (b x)} (Hmono : is_mono f) :
+    ∀ n, karrow_rel (fix_fuel n f) (fix f) := by
+    intros n x
+    simp [result_rel]
+    have Hl := least_spec (fix_fuel_P f x)
+    simp at Hl
+    match Hl with
+    | .inl Hl => simp [*]
+    | .inr ⟨ Hl, Hn ⟩ =>
+      match Classical.em (fix_fuel n f x = div) with
+      | .inl Hd =>
+        simp [*]
+      | .inr Hd =>
+        have Hineq : least (fix_fuel_P f x) ≤ n := by
+          -- Proof by contradiction
+          cases Classical.em (least (fix_fuel_P f x) ≤ n) <;> simp [*]
+          simp at *
+          rename_i Hineq
+          have Hn := Hn n Hineq
+          contradiction
+        have Hfix : ¬ (fix f x = div) := by
+          simp [fix]
+          -- By property of the least upper bound
+          revert Hd Hl
+          -- TODO: there is no conversion to select the head of a function!
+          conv => lhs; apply congr_fun; apply congr_fun; apply congr_fun; simp [fix_fuel_P, div?]
+          cases fix_fuel (least (fix_fuel_P f x)) f x <;> simp
+        have Hmono := fix_fuel_mono Hmono Hineq x
+        simp [result_rel] at Hmono
+        simp [fix] at *
+        cases Heq: fix_fuel (least (fix_fuel_P f x)) f x <;>
+        cases Heq':fix_fuel n f x <;>
+        simp_all
+
+  theorem fix_fuel_P_least {f : ((x:a) → Result (b x)) → (x:a) → Result (b x)} (Hmono : is_mono f) :
+    ∀ {{x n}}, fix_fuel_P f x n → fix_fuel_P f x (least (fix_fuel_P f x)) := by
+    intros x n Hf
+    have Hfmono := fix_fuel_fix_mono Hmono n x
+    -- TODO: there is no conversion to select the head of a function!
+    conv => apply congr_fun; simp [fix_fuel_P]
+    simp [fix_fuel_P] at Hf
+    revert Hf Hfmono
+    simp [div?, result_rel, fix]
+    cases fix_fuel n f x <;> simp_all
+
+  -- Prove the fixed point equation in the case there exists some fuel for which
+  -- the execution terminates
+  theorem fix_fixed_eq_terminates (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (Hmono : is_mono f)
+    (x : a) (n : Nat) (He : fix_fuel_P f x n) :
+    fix f x = f (fix f) x := by
+    have Hl := fix_fuel_P_least Hmono He
+    -- TODO: better control of simplification
+    conv at Hl =>
+      apply congr_fun
+      simp [fix_fuel_P]
+    -- The least upper bound is > 0
+    have ⟨ n, Hsucc ⟩ : ∃ n, least (fix_fuel_P f x) = Nat.succ n := by
+      revert Hl
+      simp [div?]
+      cases least (fix_fuel_P f x) <;> simp [fix_fuel]
+    simp [Hsucc] at Hl
+    revert Hl
+    simp [*, div?, fix, fix_fuel]
+    -- Use the monotonicity
+    have Hfixmono := fix_fuel_fix_mono Hmono n
+    have Hvm := Hmono Hfixmono x
+    -- Use functional extensionality
+    simp [result_rel, fix] at Hvm
+    revert Hvm
+    split <;> simp [*] <;> intros <;> simp [*]
+
+  theorem fix_fixed_eq_forall {{f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
+    (Hmono : is_mono f) (Hcont : is_cont f) :
+    ∀ x, fix f x = f (fix f) x := by
+    intros x
+    -- Case disjunction: is there a fuel such that the execution successfully execute?
+    match Classical.em (∃ n, fix_fuel_P f x n) with
+    | .inr He =>
+      -- No fuel: the fixed point evaluates to `div`
+      --simp [fix] at *
+      simp at *
+      conv => lhs; simp [fix]
+      have Hel := He (Nat.succ (least (fix_fuel_P f x))); simp [*, fix_fuel] at *; clear Hel
+      -- Use the "continuity" of `f`
+      have He : ∀ n, fix_fuel (.succ n) f x = div := by intros; simp [*]
+      have Hcont := Hcont x He
+      simp [Hcont]
+    | .inl ⟨ n, He ⟩ => apply fix_fixed_eq_terminates f Hmono x n He
+
+  -- The final fixed point equation
+  theorem fix_fixed_eq {{f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
+    (Hmono : is_mono f) (Hcont : is_cont f) :
+    fix f = f (fix f) := by
+    have Heq := fix_fixed_eq_forall Hmono Hcont
+    have Heq1 : fix f = (λ x => fix f x) := by simp
+    rw [Heq1]
+    conv => lhs; ext; simp [Heq]
+
+  /-! # Making the proofs of validity manageable (and automatable) -/
+
+  -- Monotonicity property for expressions
+  def is_mono_p (e : ((x:a) → Result (b x)) → Result c) : Prop :=
+    ∀ {{k1 k2}}, karrow_rel k1 k2 → result_rel (e k1) (e k2)
+
+  theorem is_mono_p_same (x : Result c) :
+    @is_mono_p a b c (λ _ => x) := by
+    simp [is_mono_p, karrow_rel, result_rel]
+    split <;> simp
+
+  theorem is_mono_p_rec (x : a) :
+    @is_mono_p a b (b x) (λ f => f x) := by
+    simp_all [is_mono_p, karrow_rel, result_rel]
+
+  -- The important lemma about `is_mono_p`
+  theorem is_mono_p_bind
+    (g : ((x:a) → Result (b x)) → Result c)
+    (h : c → ((x:a) → Result (b x)) → Result d) :
+    is_mono_p g →
+    (∀ y, is_mono_p (h y)) →
+    @is_mono_p a b d (λ k => do let y ← g k; h y k) := by
+    intro hg hh
+    simp [is_mono_p]
+    intro fg fh Hrgh
+    simp [karrow_rel, result_rel]
+    have hg := hg Hrgh; simp [result_rel] at hg
+    cases heq0: g fg <;> simp_all
+    rename_i y _
+    have hh := hh y Hrgh; simp [result_rel] at hh
+    simp_all
+
+  -- Continuity property for expressions - note that we take the continuation
+  -- as parameter
+  def is_cont_p (k : ((x:a) → Result (b x)) → (x:a) → Result (b x))
+    (e : ((x:a) → Result (b x)) → Result c) : Prop :=
+    (Hc : ∀ n, e (fix_fuel n k) = .div) →
+    e (fix k) = .div
+
+  theorem is_cont_p_same (k : ((x:a) → Result (b x)) → (x:a) → Result (b x))
+    (x : Result c) :
+    is_cont_p k (λ _ => x) := by
+    simp [is_cont_p]
+
+  theorem is_cont_p_rec (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : a) :
+    is_cont_p f (λ f => f x) := by
+    simp_all [is_cont_p, fix]
+
+  -- The important lemma about `is_cont_p`
+  theorem is_cont_p_bind
+    (k : ((x:a) → Result (b x)) → (x:a) → Result (b x))
+    (Hkmono : is_mono k)
+    (g : ((x:a) → Result (b x)) → Result c)
+    (h : c → ((x:a) → Result (b x)) → Result d) :
+    is_mono_p g →
+    is_cont_p k g →
+    (∀ y, is_mono_p (h y)) →
+    (∀ y, is_cont_p k (h y)) →
+    is_cont_p k (λ k => do let y ← g k; h y k) := by
+    intro Hgmono Hgcont Hhmono Hhcont
+    simp [is_cont_p]
+    intro Hdiv
+    -- Case on `g (fix... k)`: is there an n s.t. it terminates?
+    cases Classical.em (∀ n, g (fix_fuel n k) = .div) <;> rename_i Hn
+    . -- Case 1: g diverges
+      have Hgcont := Hgcont Hn
+      simp_all
+    . -- Case 2: g doesn't diverge
+      simp at Hn
+      let ⟨ n, Hn ⟩ := Hn
+      have Hdivn := Hdiv n
+      have Hffmono := fix_fuel_fix_mono Hkmono n
+      have Hgeq := Hgmono Hffmono
+      simp [result_rel] at Hgeq
+      cases Heq: g (fix_fuel n k) <;> rename_i y <;> simp_all
+      -- Remains the .ret case
+      -- Use Hdiv to prove that: ∀ n, h y (fix_fuel n f) = div
+      -- We do this in two steps: first we prove it for m ≥ n
+      have Hhdiv: ∀ m, h y (fix_fuel m k) = .div := by
+        have Hhdiv : ∀ m, n ≤ m → h y (fix_fuel m k) = .div := by
+          -- We use the fact that `g (fix_fuel n f) = .div`, combined with Hdiv
+          intro m Hle
+          have Hdivm := Hdiv m
+          -- Monotonicity of g
+          have Hffmono := fix_fuel_mono Hkmono Hle
+          have Hgmono := Hgmono Hffmono
+          -- We need to clear Hdiv because otherwise simp_all rewrites Hdivm with Hdiv
+          clear Hdiv
+          simp_all [result_rel]
+        intro m
+        -- TODO: we shouldn't need the excluded middle here because it is decidable
+        cases Classical.em (n ≤ m) <;> rename_i Hl
+        . apply Hhdiv; assumption
+        . simp at Hl
+          -- Make a case disjunction on `h y (fix_fuel m k)`: if it is not equal
+          -- to div, use the monotonicity of `h y`
+          have Hle : m ≤ n := by linarith
+          have Hffmono := fix_fuel_mono Hkmono Hle
+          have Hmono := Hhmono y Hffmono
+          simp [result_rel] at Hmono
+          cases Heq: h y (fix_fuel m k) <;> simp_all
+      -- We can now use the continuity hypothesis for h
+      apply Hhcont; assumption
+
+  -- The validity property for an expression
+  def is_valid_p (k : ((x:a) → Result (b x)) → (x:a) → Result (b x))
+    (e : ((x:a) → Result (b x)) → Result c) : Prop :=
+    is_mono_p e ∧
+    (is_mono k → is_cont_p k e)
+
+  @[simp] theorem is_valid_p_same
+    (k : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : Result c) :
+    is_valid_p k (λ _ => x) := by
+    simp [is_valid_p, is_mono_p_same, is_cont_p_same]
+
+  @[simp] theorem is_valid_p_rec
+    (k : ((x:a) → Result (b x)) → (x:a) → Result (b x)) (x : a) :
+    is_valid_p k (λ k => k x) := by
+    simp_all [is_valid_p, is_mono_p_rec, is_cont_p_rec]
+
+  -- Lean is good at unification: we can write a very general version
+  -- (in particular, it will manage to figure out `g` and `h` when we
+  -- apply the lemma)
+  theorem is_valid_p_bind
+    {{k : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
+    {{g : ((x:a) → Result (b x)) → Result c}}
+    {{h : c → ((x:a) → Result (b x)) → Result d}}
+    (Hgvalid : is_valid_p k g)
+    (Hhvalid : ∀ y, is_valid_p k (h y)) :
+    is_valid_p k (λ k => do let y ← g k; h y k) := by
+    let ⟨ Hgmono, Hgcont ⟩ := Hgvalid
+    simp [is_valid_p, forall_and] at Hhvalid
+    have ⟨ Hhmono, Hhcont ⟩ := Hhvalid
+    simp [← imp_forall_iff] at Hhcont
+    simp [is_valid_p]; constructor
+    . -- Monotonicity
+      apply is_mono_p_bind <;> assumption
+    . -- Continuity
+      intro Hkmono
+      have Hgcont := Hgcont Hkmono
+      have Hhcont := Hhcont Hkmono
+      apply is_cont_p_bind <;> assumption
+
+  def is_valid (f : ((x:a) → Result (b x)) → (x:a) → Result (b x)) : Prop :=
+    ∀ k x, is_valid_p k (λ k => f k x)
+
+  theorem is_valid_p_imp_is_valid {{f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
+    (Hvalid : is_valid f) :
+    is_mono f ∧ is_cont f := by
+    have Hmono : is_mono f := by
+      intro f h Hr x
+      have Hmono := Hvalid (λ _ _ => .div) x
+      have Hmono := Hmono.left
+      apply Hmono; assumption
+    have Hcont : is_cont f := by
+      intro x Hdiv
+      have Hcont := (Hvalid f x).right Hmono
+      simp [is_cont_p] at Hcont
+      apply Hcont
+      intro n
+      have Hdiv := Hdiv n
+      simp [fix_fuel] at Hdiv
+      simp [*]
+    simp [*]
+
+  theorem is_valid_fix_fixed_eq {{f : ((x:a) → Result (b x)) → (x:a) → Result (b x)}}
+    (Hvalid : is_valid f) :
+    fix f = f (fix f) := by
+    have ⟨ Hmono, Hcont ⟩ := is_valid_p_imp_is_valid Hvalid
+    exact fix_fixed_eq Hmono Hcont
+
+end Fix
+
+namespace FixI
+  /- Indexed fixed-point: definitions with indexed types, convenient to use for mutually
+     recursive definitions. We simply port the definitions and proofs from Fix to a more
+     specific case.
+   -/
+  open Primitives Fix
+
+  -- The index type
+  variable {id : Type}
+
+  -- The input/output types
+  variable {a b : id → Type}
+
+  -- Monotonicity relation over monadic arrows (i.e., Kleisli arrows)
+  def karrow_rel (k1 k2 : (i:id) → a i → Result (b i)) : Prop :=
+    ∀ i x, result_rel (k1 i x) (k2 i x)
+
+  def kk_to_gen (k : (i:id) → a i → Result (b i)) :
+    (x: (i:id) × a i) → Result (b x.fst) :=
+    λ ⟨ i, x ⟩ => k i x
+
+  def kk_of_gen (k : (x: (i:id) × a i) → Result (b x.fst)) :
+    (i:id) → a i → Result (b i) :=
+    λ i x => k ⟨ i, x ⟩
+
+  def k_to_gen (k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) :
+    ((x: (i:id) × a i) → Result (b x.fst)) → (x: (i:id) × a i) → Result (b x.fst) :=
+    λ kk => kk_to_gen (k (kk_of_gen kk))
+
+  def k_of_gen (k : ((x: (i:id) × a i) → Result (b x.fst)) → (x: (i:id) × a i) → Result (b x.fst)) :
+    ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i) :=
+    λ kk => kk_of_gen (k (kk_to_gen kk))
+
+  def e_to_gen (e : ((i:id) → a i → Result (b i)) → Result c) :
+    ((x: (i:id) × a i) → Result (b x.fst)) → Result c :=
+    λ k => e (kk_of_gen k)
+
+  def is_valid_p (k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i))
+    (e : ((i:id) → a i → Result (b i)) → Result c) : Prop :=
+    Fix.is_valid_p (k_to_gen k) (e_to_gen e)
+
+  def is_valid (f : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) : Prop :=
+    ∀ k i x, is_valid_p k (λ k => f k i x)
+
+  noncomputable def fix
+    (f : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) :
+    (i:id) → a i → Result (b i) :=
+    kk_of_gen (Fix.fix (k_to_gen f))
+
+  theorem is_valid_fix_fixed_eq
+    {{f : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)}}
+    (Hvalid : is_valid f) :
+    fix f = f (fix f) := by
+    have Hvalid' : Fix.is_valid (k_to_gen f) := by
+      intro k x
+      simp only [is_valid, is_valid_p] at Hvalid
+      let ⟨ i, x ⟩ := x
+      have Hvalid := Hvalid (k_of_gen k) i x
+      simp only [k_to_gen, k_of_gen, kk_to_gen, kk_of_gen] at Hvalid
+      refine Hvalid
+    have Heq := Fix.is_valid_fix_fixed_eq Hvalid'
+    simp [fix]
+    conv => lhs; rw [Heq]
+
+  /- Some utilities to define the mutually recursive functions -/
+
+  -- TODO: use more
+  @[simp] def kk_ty (id : Type) (a b : id → Type) := (i:id) → a i → Result (b i)
+  @[simp] def k_ty (id : Type) (a b : id → Type) := kk_ty id a b → kk_ty id a b
+
+  -- Initially, we had left out the parameters id, a and b.
+  -- However, by parameterizing Funs with those parameters, we can state
+  -- and prove lemmas like Funs.is_valid_p_is_valid_p
+  inductive Funs (id : Type) (a b : id → Type) :
+    List (Type u) → List (Type u) → Type (u + 1) :=
+    | Nil : Funs id a b [] []
+    | Cons {ity oty : Type u} {itys otys : List (Type u)}
+           (f : kk_ty id a b → ity → Result oty) (tl : Funs id a b itys otys) :
+           Funs id a b (ity :: itys) (oty :: otys)
+
+  theorem Funs.length_eq {itys otys : List (Type)} (fl : Funs id a b itys otys) :
+    otys.length = itys.length :=
+    match fl with
+    | .Nil => by simp
+    | .Cons f tl =>
+      have h:= Funs.length_eq tl
+      by simp [h]
+
+  def fin_cast {n m : Nat} (h : m = n) (i : Fin n) : Fin m :=
+    ⟨ i.val, by have h1:= i.isLt; simp_all ⟩
+
+  @[simp] def Funs.cast_fin {itys otys : List (Type)}
+    (fl : Funs id a b itys otys) (i : Fin itys.length) : Fin otys.length :=
+    fin_cast (fl.length_eq) i
+
+  def get_fun {itys otys : List (Type)} (fl : Funs id a b itys otys) :
+    (i : Fin itys.length) → kk_ty id a b → itys.get i → Result (otys.get (fl.cast_fin i)) :=
+    match fl with
+    | .Nil => λ i => by have h:= i.isLt; simp at h
+    | @Funs.Cons id a b ity oty itys1 otys1 f tl =>
+      λ i =>
+      if h: i.val = 0 then
+        Eq.mp (by cases i; simp_all [List.get]) f
+      else
+        let j := i.val - 1
+        have Hj: j < itys1.length := by
+          have Hi := i.isLt
+          simp at Hi
+          revert Hi
+          cases Heq: i.val <;> simp_all
+          simp_arith
+        let j: Fin itys1.length := ⟨ j, Hj ⟩
+        Eq.mp
+          (by
+            cases Heq: i; rename_i val isLt;
+            cases Heq': j; rename_i val' isLt;
+            cases val <;> simp_all [List.get, fin_cast])
+          (get_fun tl j)
+
+  -- TODO: move
+  theorem add_one_le_iff_le_ne (n m : Nat) (h1 : m ≤ n) (h2 : m ≠ n) : m + 1 ≤ n := by
+    -- Damn, those proofs on natural numbers are hard - I wish Omega was in mathlib4...
+    simp [Nat.add_one_le_iff]
+    simp [Nat.lt_iff_le_and_ne]
+    simp_all
+
+  def for_all_fin_aux {n : Nat} (f : Fin n → Prop) (m : Nat) (h : m ≤ n) : Prop :=
+    if heq: m = n then True
+    else
+      f ⟨ m, by simp_all [Nat.lt_iff_le_and_ne] ⟩ ∧
+      for_all_fin_aux f (m + 1) (by simp_all [add_one_le_iff_le_ne])
+  termination_by for_all_fin_aux n _ m h => n - m
+  decreasing_by
+    simp_wf
+    apply Nat.sub_add_lt_sub <;> simp
+    simp_all [add_one_le_iff_le_ne]
+
+  def for_all_fin {n : Nat} (f : Fin n → Prop) := for_all_fin_aux f 0 (by simp)
+
+  theorem for_all_fin_aux_imp_forall {n : Nat} (f : Fin n → Prop) (m : Nat) :
+    (h : m ≤ n) →
+    for_all_fin_aux f m h → ∀ i, m ≤ i.val → f i
+    := by
+    generalize h: (n - m) = k
+    revert m
+    induction k -- TODO: induction h rather?
+    case zero =>
+      simp_all
+      intro m h1 h2
+      have h: n = m := by
+        linarith
+      unfold for_all_fin_aux; simp_all
+      simp_all
+      -- There is no i s.t. m ≤ i
+      intro i h3; cases i; simp_all
+      linarith
+    case succ k hi =>
+      simp_all
+      intro m hk hmn
+      intro hf i hmi
+      have hne: m ≠ n := by
+        have hineq := Nat.lt_of_sub_eq_succ hk
+        linarith
+      -- m = i?
+      if heq: m = i then
+        -- Yes: simply use the `for_all_fin_aux` hyp
+        unfold for_all_fin_aux at hf
+        simp_all
+        tauto
+      else
+        -- No: use the induction hypothesis
+        have hlt: m < i := by simp_all [Nat.lt_iff_le_and_ne]
+        have hineq: m + 1 ≤ n := by
+          have hineq := Nat.lt_of_sub_eq_succ hk
+          simp [*, Nat.add_one_le_iff]
+        have heq1: n - (m + 1) = k := by
+          -- TODO: very annoying arithmetic proof
+          simp [Nat.sub_eq_iff_eq_add hineq]
+          have hineq1: m ≤ n := by linarith
+          simp [Nat.sub_eq_iff_eq_add hineq1] at hk
+          simp_arith [hk]
+        have hi := hi (m + 1) heq1 hineq
+        apply hi <;> simp_all
+        . unfold for_all_fin_aux at hf
+          simp_all
+        . simp_all [add_one_le_iff_le_ne]
+
+  -- TODO: this is not necessary anymore
+  theorem for_all_fin_imp_forall (n : Nat) (f : Fin n → Prop) :
+    for_all_fin f → ∀ i, f i
+    := by
+    intro Hf i
+    apply for_all_fin_aux_imp_forall <;> try assumption
+    simp
+
+  /- Automating the proofs -/
+  @[simp] theorem is_valid_p_same
+    (k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) (x : Result c) :
+    is_valid_p k (λ _ => x) := by
+    simp [is_valid_p, k_to_gen, e_to_gen]
+
+  @[simp] theorem is_valid_p_rec
+     (k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)) (i : id) (x : a i) :
+    is_valid_p k (λ k => k i x) := by
+    simp [is_valid_p, k_to_gen, e_to_gen, kk_to_gen, kk_of_gen]
+
+  theorem is_valid_p_bind
+    {{k : ((i:id) → a i → Result (b i)) → (i:id) → a i → Result (b i)}}
+    {{g : ((i:id) → a i → Result (b i)) → Result c}}
+    {{h : c → ((i:id) → a i → Result (b i)) → Result d}}
+    (Hgvalid : is_valid_p k g)
+    (Hhvalid : ∀ y, is_valid_p k (h y)) :
+    is_valid_p k (λ k => do let y ← g k; h y k) := by
+    apply Fix.is_valid_p_bind
+    . apply Hgvalid
+    . apply Hhvalid
+
+  def Funs.is_valid_p
+    (k : k_ty id a b)
+    (fl : Funs id a b itys otys) :
+    Prop :=
+    match fl with
+    | .Nil => True
+    | .Cons f fl => (∀ x, FixI.is_valid_p k (λ k => f k x)) ∧ fl.is_valid_p k
+
+  def Funs.is_valid_p_is_valid_p_aux
+    {k : k_ty id a b}
+    {itys otys : List Type}
+    (Heq : List.length otys = List.length itys)
+    (fl : Funs id a b itys otys) (Hvalid : is_valid_p k fl) :
+    ∀ (i : Fin itys.length) (x : itys.get i), FixI.is_valid_p k (fun k => get_fun fl i k x) := by
+    -- Prepare the induction
+    have ⟨ n, Hn ⟩ : { n : Nat // itys.length = n } := ⟨ itys.length, by rfl ⟩
+    revert itys otys Heq fl Hvalid
+    induction n
+    --
+    case zero =>
+      intro itys otys Heq fl Hvalid Hlen;
+      have Heq: itys = [] := by cases itys <;> simp_all
+      have Heq: otys = [] := by cases otys <;> simp_all
+      intro i x
+      simp_all
+      have Hi := i.isLt
+      simp_all
+    case succ n Hn =>
+      intro itys otys Heq fl Hvalid Hlen i x;
+      cases fl <;> simp at Hlen i x Heq Hvalid
+      rename_i ity oty itys otys f fl
+      have ⟨ Hvf, Hvalid ⟩ := Hvalid
+      have Hvf1: is_valid_p k fl := by
+        simp [Hvalid, Funs.is_valid_p]
+      have Hn := @Hn itys otys (by simp[*]) fl Hvf1 (by simp [*])
+      -- Case disjunction on i
+      match i with
+      | ⟨ 0, _ ⟩ =>
+        simp at x
+        simp [get_fun]
+        apply (Hvf x)
+      | ⟨ .succ j, HiLt ⟩ =>
+        simp_arith at HiLt
+        simp at x
+        let j : Fin (List.length itys) := ⟨ j, by simp_arith [HiLt] ⟩
+        have Hn := Hn j x
+        apply Hn
+
+  def Funs.is_valid_p_is_valid_p
+    (itys otys : List (Type)) (Heq: otys.length = itys.length := by decide)
+    (k : k_ty (Fin (List.length itys)) (List.get itys) fun i => List.get otys (fin_cast Heq i))
+    (fl : Funs (Fin itys.length) itys.get (λ i => otys.get (fin_cast Heq i)) itys otys) :
+    fl.is_valid_p k  →
+    ∀ (i : Fin itys.length) (x : itys.get i), FixI.is_valid_p k (fun k => get_fun fl i k x)
+    := by
+    intro Hvalid
+    apply is_valid_p_is_valid_p_aux <;> simp [*]
+
+end FixI
+
+namespace Ex1
+  /- An example of use of the fixed-point -/
+  open Primitives Fix
+
+  variable {a : Type} (k : (List a × Int) → Result a)
+
+  def list_nth_body (x : (List a × Int)) : Result a :=
+    let (ls, i) := x
+    match ls with
+    | [] => .fail .panic
+    | hd :: tl =>
+      if i = 0 then .ret hd
+      else k (tl, i - 1)
+
+  theorem list_nth_body_is_valid: ∀ k x, is_valid_p k (λ k => @list_nth_body a k x) := by
+    intro k x
+    simp [list_nth_body]
+    split <;> simp
+    split <;> simp
+
+  noncomputable
+  def list_nth (ls : List a) (i : Int) : Result a := fix list_nth_body (ls, i)
+
+  -- The unfolding equation - diverges if `i < 0`
+  theorem list_nth_eq (ls : List a) (i : Int) :
+    list_nth ls i =
+      match ls with
+      | [] => .fail .panic
+      | hd :: tl =>
+        if i = 0 then .ret hd
+        else list_nth tl (i - 1)
+    := by
+    have Heq := is_valid_fix_fixed_eq (@list_nth_body_is_valid a)
+    simp [list_nth]
+    conv => lhs; rw [Heq]
+
+end Ex1
+
+namespace Ex2
+  /- Same as Ex1, but we make the body of nth non tail-rec (this is mostly
+     to see what happens when there are let-bindings) -/
+  open Primitives Fix
+
+  variable {a : Type} (k : (List a × Int) → Result a)
+
+  def list_nth_body (x : (List a × Int)) : Result a :=
+    let (ls, i) := x
+    match ls with
+    | [] => .fail .panic
+    | hd :: tl =>
+      if i = 0 then .ret hd
+      else
+        do
+          let y ← k (tl, i - 1)
+          .ret y
+
+  theorem list_nth_body_is_valid: ∀ k x, is_valid_p k (λ k => @list_nth_body a k x) := by
+    intro k x
+    simp [list_nth_body]
+    split <;> simp
+    split <;> simp
+    apply is_valid_p_bind <;> intros <;> simp_all
+
+  noncomputable
+  def list_nth (ls : List a) (i : Int) : Result a := fix list_nth_body (ls, i)
+
+  -- The unfolding equation - diverges if `i < 0`
+  theorem list_nth_eq (ls : List a) (i : Int) :
+    (list_nth ls i =
+       match ls with
+       | [] => .fail .panic
+       | hd :: tl =>
+         if i = 0 then .ret hd
+         else
+           do
+             let y ← list_nth tl (i - 1)
+             .ret y)
+    := by
+    have Heq := is_valid_fix_fixed_eq (@list_nth_body_is_valid a)
+    simp [list_nth]
+    conv => lhs; rw [Heq]
+
+end Ex2
+
+namespace Ex3
+  /- Mutually recursive functions - first encoding (see Ex4 for a better encoding) -/
+  open Primitives Fix
+
+  /- Because we have mutually recursive functions, we use a sum for the inputs
+     and the output types:
+     - inputs: the sum allows to select the function to call in the recursive
+       calls (and the functions may not have the same input types)
+     - outputs: this case is degenerate because `even` and `odd` have the same
+       return type `Bool`, but generally speaking we need a sum type because
+       the functions in the mutually recursive group may have different
+       return types.
+   -/
+  variable (k : (Int ⊕ Int) → Result (Bool ⊕ Bool))
+
+  def is_even_is_odd_body (x : (Int ⊕ Int)) : Result (Bool ⊕ Bool) :=
+    match x with
+    | .inl i =>
+      -- Body of `is_even`
+      if i = 0
+      then .ret (.inl true) -- We use .inl because this is `is_even`
+      else
+        do
+          let b ←
+            do
+              -- Call `odd`: we need to wrap the input value in `.inr`, then
+              -- extract the output value
+              let r ← k (.inr (i- 1))
+              match r with
+              | .inl _ => .fail .panic -- Invalid output
+              | .inr b => .ret b
+          -- Wrap the return value
+          .ret (.inl b)
+    | .inr i =>
+      -- Body of `is_odd`
+      if i = 0
+      then .ret (.inr false) -- We use .inr because this is `is_odd`
+      else
+        do
+          let b ←
+            do
+              -- Call `is_even`: we need to wrap the input value in .inr, then
+              -- extract the output value
+              let r ← k (.inl (i- 1))
+              match r with
+              | .inl b => .ret b
+              | .inr _ => .fail .panic -- Invalid output
+          -- Wrap the return value
+          .ret (.inr b)
+
+  theorem is_even_is_odd_body_is_valid:
+    ∀ k x, is_valid_p k (λ k => is_even_is_odd_body k x) := by
+    intro k x
+    simp [is_even_is_odd_body]
+    split <;> simp <;> split <;> simp
+    apply is_valid_p_bind; simp
+    intros; split <;> simp
+    apply is_valid_p_bind; simp
+    intros; split <;> simp
+
+  noncomputable
+  def is_even (i : Int): Result Bool :=
+    do
+      let r ← fix is_even_is_odd_body (.inl i)
+      match r with
+      | .inl b => .ret b
+      | .inr _ => .fail .panic
+
+  noncomputable
+  def is_odd (i : Int): Result Bool :=
+    do
+      let r ← fix is_even_is_odd_body (.inr i)
+      match r with
+      | .inl _ => .fail .panic
+      | .inr b => .ret b
+
+  -- The unfolding equation for `is_even` - diverges if `i < 0`
+  theorem is_even_eq (i : Int) :
+    is_even i = (if i = 0 then .ret true else is_odd (i - 1))
+    := by
+    have Heq := is_valid_fix_fixed_eq is_even_is_odd_body_is_valid
+    simp [is_even, is_odd]
+    conv => lhs; rw [Heq]; simp; rw [is_even_is_odd_body]; simp
+    -- Very annoying: we need to swap the matches
+    -- Doing this with rewriting lemmas is hard generally speaking
+    -- (especially as we may have to generate lemmas for user-defined
+    -- inductives on the fly).
+    -- The simplest is to repeatedly split then simplify (we identify
+    -- the outer match or monadic let-binding, and split on its scrutinee)
+    split <;> simp
+    cases H0 : fix is_even_is_odd_body (Sum.inr (i - 1)) <;> simp
+    rename_i v
+    split <;> simp
+
+  -- The unfolding equation for `is_odd` - diverges if `i < 0`
+  theorem is_odd_eq (i : Int) :
+    is_odd i = (if i = 0 then .ret false else is_even (i - 1))
+    := by
+    have Heq := is_valid_fix_fixed_eq is_even_is_odd_body_is_valid
+    simp [is_even, is_odd]
+    conv => lhs; rw [Heq]; simp; rw [is_even_is_odd_body]; simp
+    -- Same remark as for `even`
+    split <;> simp
+    cases H0 : fix is_even_is_odd_body (Sum.inl (i - 1)) <;> simp
+    rename_i v
+    split <;> simp
+
+end Ex3
+
+namespace Ex4
+  /- Mutually recursive functions - 2nd encoding -/
+  open Primitives FixI
+
+  attribute [local simp] List.get
+
+  /- We make the input type and output types dependent on a parameter -/
+  @[simp] def input_ty (i : Fin 2) : Type :=
+    [Int, Int].get i
+
+  @[simp] def output_ty (i : Fin 2) : Type :=
+    [Bool, Bool].get i
+
+  /- The continuation -/
+  variable (k : (i : Fin 2) → input_ty i → Result (output_ty i))
+
+  /- The bodies are more natural -/
+  def is_even_body (k : (i : Fin 2) → input_ty i → Result (output_ty i)) (i : Int) : Result Bool :=
+    if i = 0
+      then .ret true
+    else do
+      let b ← k 1 (i - 1)
+      .ret b
+
+  def is_odd_body (i : Int) : Result Bool :=
+    if i = 0
+      then .ret false
+    else do
+      let b ← k 0 (i - 1)
+      .ret b
+
+  @[simp] def bodies :
+    Funs (Fin 2) input_ty output_ty [Int, Int] [Bool, Bool] :=
+    Funs.Cons (is_even_body) (Funs.Cons (is_odd_body) Funs.Nil)
+
+  def body (k : (i : Fin 2) → input_ty i → Result (output_ty i)) (i: Fin 2) :
+    input_ty i → Result (output_ty i) := get_fun bodies i k
+
+  theorem body_is_valid : is_valid body := by
+    -- Split the proof into proofs of validity of the individual bodies
+    rw [is_valid]
+    simp only [body]
+    intro k
+    apply (Funs.is_valid_p_is_valid_p [Int, Int] [Bool, Bool])
+    simp [Funs.is_valid_p]
+    (repeat (apply And.intro)) <;> intro x <;> simp at x <;>
+    simp only [is_even_body, is_odd_body]
+    -- Prove the validity of the individual bodies
+    . split <;> simp
+      apply is_valid_p_bind <;> simp
+    . split <;> simp
+      apply is_valid_p_bind <;> simp
+
+  theorem body_fix_eq : fix body = body (fix body) :=
+    is_valid_fix_fixed_eq body_is_valid
+
+  noncomputable def is_even (i : Int) : Result Bool := fix body 0 i
+  noncomputable def is_odd (i : Int) : Result Bool := fix body 1 i
+
+  theorem is_even_eq (i : Int) : is_even i =
+    (if i = 0
+       then .ret true
+     else do
+       let b ← is_odd (i - 1)
+       .ret b) := by
+    simp [is_even, is_odd];
+    conv => lhs; rw [body_fix_eq]
+
+  theorem is_odd_eq (i : Int) : is_odd i =
+    (if i = 0
+       then .ret false
+     else do
+       let b ← is_even (i - 1)
+       .ret b) := by
+    simp [is_even, is_odd];
+    conv => lhs; rw [body_fix_eq]
+
+end Ex4
+
+namespace Ex5
+  /- Higher-order example -/
+  open Primitives Fix
+
+  variable {a b : Type}
+
+  /- An auxiliary function, which doesn't require the fixed-point -/
+  def map (f : a → Result b) (ls : List a) : Result (List b) :=
+    match ls with
+    | [] => .ret []
+    | hd :: tl =>
+      do
+        let hd ← f hd
+        let tl ← map f tl
+        .ret (hd :: tl)
+
+  /- The validity theorem for `map`, generic in `f` -/
+  theorem map_is_valid
+    {{f : (a → Result b) → a → Result c}}
+    (Hfvalid : ∀ k x, is_valid_p k (λ k => f k x))
+    (k : (a → Result b) → a → Result b)
+    (ls : List a) :
+    is_valid_p k (λ k => map (f k) ls) := by
+    induction ls <;> simp [map]
+    apply is_valid_p_bind <;> simp_all
+    intros
+    apply is_valid_p_bind <;> simp_all
+
+  /- An example which uses map -/
+  inductive Tree (a : Type) :=
+  | leaf (x : a)
+  | node (tl : List (Tree a))
+
+  def id_body (k : Tree a → Result (Tree a)) (t : Tree a) : Result (Tree a) :=
+    match t with
+    | .leaf x => .ret (.leaf x)
+    | .node tl =>
+      do
+        let tl ← map k tl
+        .ret (.node tl)
+
+  theorem id_body_is_valid :
+    ∀ k x, is_valid_p k (λ k => @id_body a k x) := by
+    intro k x
+    simp only [id_body]
+    split <;> simp
+    apply is_valid_p_bind <;> simp [*]
+    -- We have to show that `map k tl` is valid
+    apply map_is_valid;
+    -- Remark: if we don't do the intro, then the last step is expensive:
+    -- "typeclass inference of Nonempty took 119ms"
+    intro k x
+    simp only [is_valid_p_same, is_valid_p_rec]
+
+  noncomputable def id (t : Tree a) := fix id_body t
+
+  -- The unfolding equation
+  theorem id_eq (t : Tree a) :
+    (id t =
+       match t with
+       | .leaf x => .ret (.leaf x)
+       | .node tl =>
+       do
+         let tl ← map id tl
+         .ret (.node tl))
+    := by
+    have Heq := is_valid_fix_fixed_eq (@id_body_is_valid a)
+    simp [id]
+    conv => lhs; rw [Heq]; simp; rw [id_body]
+
+end Ex5
diff --git a/backends/lean/Base/Diverge/Elab.lean b/backends/lean/Base/Diverge/Elab.lean
new file mode 100644
index 00000000..313c5a79
--- /dev/null
+++ b/backends/lean/Base/Diverge/Elab.lean
@@ -0,0 +1,182 @@
+import Lean
+import Lean.Meta.Tactic.Simp
+import Init.Data.List.Basic
+import Mathlib.Tactic.RunCmd
+import Base.Diverge.Base
+import Base.Diverge.ElabBase
+
+namespace Diverge
+
+/- Automating the generation of the encoding and the proofs so as to use nice
+   syntactic sugar. -/
+
+syntax (name := divergentDef)
+  declModifiers "divergent" "def" declId ppIndent(optDeclSig) declVal : command
+
+open Lean Elab Term Meta Primitives
+
+initialize registerTraceClass `Diverge.divRecursion (inherited := true)
+
+set_option trace.Diverge.divRecursion true
+
+/- The following was copied from the `wfRecursion` function. -/
+
+open WF in
+def divRecursion (preDefs : Array PreDefinition) : TermElabM Unit := do
+  let msg := toMessageData <| preDefs.map fun pd => (pd.declName, pd.levelParams, pd.type, pd.value)
+  logInfo ("divRecursion: defs: " ++ msg)
+
+  -- CHANGE HERE This function should add definitions with these names/types/values ^^
+  -- Temporarily add the predefinitions as axioms
+  for preDef in preDefs do
+    addAsAxiom preDef
+
+  -- TODO: what is this?
+  for preDef in preDefs do
+    applyAttributesOf #[preDef] AttributeApplicationTime.afterCompilation
+
+  -- Process the definitions
+  addAndCompilePartialRec preDefs
+
+-- The following function is copy&pasted from Lean.Elab.PreDefinition.Main
+-- This is the only part where we make actual changes and hook into the equation compiler.
+-- (I've removed all the well-founded stuff to make it easier to read though.)
+
+open private ensureNoUnassignedMVarsAtPreDef betaReduceLetRecApps partitionPreDefs
+  addAndCompilePartial addAsAxioms from Lean.Elab.PreDefinition.Main
+
+def addPreDefinitions (preDefs : Array PreDefinition) : TermElabM Unit := withLCtx {} {} do
+  for preDef in preDefs do
+    trace[Elab.definition.body] "{preDef.declName} : {preDef.type} :=\n{preDef.value}"
+  let preDefs ← preDefs.mapM ensureNoUnassignedMVarsAtPreDef
+  let preDefs ← betaReduceLetRecApps preDefs
+  let cliques := partitionPreDefs preDefs
+  let mut hasErrors := false
+  for preDefs in cliques do
+    trace[Elab.definition.scc] "{preDefs.map (·.declName)}"
+    try
+      logInfo "calling divRecursion"
+      withRef (preDefs[0]!.ref) do
+        divRecursion preDefs
+      logInfo "divRecursion succeeded"
+    catch ex =>
+      -- If it failed, we 
+      logInfo "divRecursion failed"
+      hasErrors := true
+      logException ex
+      let s ← saveState
+      try
+        if preDefs.all fun preDef => preDef.kind == DefKind.def ||
+           preDefs.all fun preDef => preDef.kind == DefKind.abbrev then
+          -- try to add as partial definition
+          try
+            addAndCompilePartial preDefs (useSorry := true)
+          catch _ =>
+            -- Compilation failed try again just as axiom
+            s.restore
+            addAsAxioms preDefs
+        else return ()
+      catch _ => s.restore
+
+-- The following two functions are copy&pasted from Lean.Elab.MutualDef
+
+open private elabHeaders levelMVarToParamHeaders getAllUserLevelNames withFunLocalDecls elabFunValues
+  instantiateMVarsAtHeader instantiateMVarsAtLetRecToLift checkLetRecsToLiftTypes withUsed from Lean.Elab.MutualDef
+
+def Term.elabMutualDef (vars : Array Expr) (views : Array DefView) : TermElabM Unit := do
+    let scopeLevelNames ← getLevelNames
+    let headers ← elabHeaders views
+    let headers ← levelMVarToParamHeaders views headers
+    let allUserLevelNames := getAllUserLevelNames headers
+    withFunLocalDecls headers fun funFVars => do
+      for view in views, funFVar in funFVars do
+        addLocalVarInfo view.declId funFVar
+      let values ←
+        try
+          let values ← elabFunValues headers
+          Term.synthesizeSyntheticMVarsNoPostponing
+          values.mapM (instantiateMVars ·)
+        catch ex =>
+          logException ex
+          headers.mapM fun header => mkSorry header.type (synthetic := true)
+      let headers ← headers.mapM instantiateMVarsAtHeader
+      let letRecsToLift ← getLetRecsToLift
+      let letRecsToLift ← letRecsToLift.mapM instantiateMVarsAtLetRecToLift
+      checkLetRecsToLiftTypes funFVars letRecsToLift
+      withUsed vars headers values letRecsToLift fun vars => do
+        let preDefs ← MutualClosure.main vars headers funFVars values letRecsToLift
+        for preDef in preDefs do
+          trace[Elab.definition] "{preDef.declName} : {preDef.type} :=\n{preDef.value}"
+        let preDefs ← withLevelNames allUserLevelNames <| levelMVarToParamPreDecls preDefs
+        let preDefs ← instantiateMVarsAtPreDecls preDefs
+        let preDefs ← fixLevelParams preDefs scopeLevelNames allUserLevelNames
+        for preDef in preDefs do
+          trace[Elab.definition] "after eraseAuxDiscr, {preDef.declName} : {preDef.type} :=\n{preDef.value}"
+        checkForHiddenUnivLevels allUserLevelNames preDefs
+        addPreDefinitions preDefs
+
+open Command in
+def Command.elabMutualDef (ds : Array Syntax) : CommandElabM Unit := do
+  let views ← ds.mapM fun d => do
+    let `($mods:declModifiers divergent def $id:declId $sig:optDeclSig $val:declVal) := d
+      | throwUnsupportedSyntax
+    let modifiers ← elabModifiers mods
+    let (binders, type) := expandOptDeclSig sig
+    let deriving? := none
+    pure { ref := d, kind := DefKind.def, modifiers,
+           declId := id, binders, type? := type, value := val, deriving? }
+  runTermElabM fun vars => Term.elabMutualDef vars views
+
+-- Special command so that we don't fall back to the built-in mutual when we produce an error.
+local syntax "_divergent" Parser.Command.mutual : command
+elab_rules : command | `(_divergent mutual $decls* end) => Command.elabMutualDef decls
+
+macro_rules
+  | `(mutual $decls* end) => do
+    unless !decls.isEmpty && decls.all (·.1.getKind == ``divergentDef) do
+      Macro.throwUnsupported
+    `(command| _divergent mutual $decls* end)
+
+open private setDeclIdName from Lean.Elab.Declaration
+elab_rules : command
+  | `($mods:declModifiers divergent%$tk def $id:declId $sig:optDeclSig $val:declVal) => do
+    let (name, _) := expandDeclIdCore id
+    if (`_root_).isPrefixOf name then throwUnsupportedSyntax
+    let view := extractMacroScopes name
+    let .str ns shortName := view.name | throwUnsupportedSyntax
+    let shortName' := { view with name := shortName }.review
+    let cmd ← `(mutual $mods:declModifiers divergent%$tk def $(⟨setDeclIdName id shortName'⟩):declId $sig:optDeclSig $val:declVal end)
+    if ns matches .anonymous then
+      Command.elabCommand cmd
+    else
+      Command.elabCommand <| ← `(namespace $(mkIdentFrom id ns) $cmd end $(mkIdentFrom id ns))
+
+mutual
+  divergent def is_even (i : Int) : Result Bool :=
+    if i = 0 then return true else return (← is_odd (i - 1))
+
+  divergent def is_odd (i : Int) : Result Bool :=
+    if i = 0 then return false else return (← is_even (i - 1))
+end
+
+example (i : Int) : is_even i = .ret (i % 2 = 0) ∧ is_odd i = .ret (i % 2 ≠ 0) := by
+  induction i
+  unfold is_even
+  sorry    
+
+divergent def list_nth {a: Type} (ls : List a) (i : Int) : Result a :=
+  match ls with
+  | [] => .fail .panic
+  | x :: ls =>
+    if i = 0 then return x
+    else return (← list_nth ls (i - 1))
+
+mutual
+  divergent def foo (i : Int) : Result Nat :=
+    if i > 10 then return (← foo (i / 10)) + (← bar i) else bar 10
+
+  divergent def bar (i : Int) : Result Nat :=
+    if i > 20 then foo (i / 20) else .ret 42
+end
+
+end Diverge
diff --git a/backends/lean/Base/Diverge/ElabBase.lean b/backends/lean/Base/Diverge/ElabBase.lean
new file mode 100644
index 00000000..e693dce2
--- /dev/null
+++ b/backends/lean/Base/Diverge/ElabBase.lean
@@ -0,0 +1,9 @@
+import Lean
+
+namespace Diverge
+
+open Lean
+
+initialize registerTraceClass `Diverge.divRecursion (inherited := true)
+
+end Diverge