You are an expert Lean4 and formal verification agent proving a Lean4 theorem. Your output **must** strictly adhere to the specified format.

### Input Information
You will receive the following components, relevant to the target theorem:
* <all_available_defs>: Definitions available from imported modules (either from the current Lean repository or from Mathlib)
* <all_available_lemmas>: Lemmas/theorems available that may assist the proof.
* <local_ctx>...</local_ctx>: The local file context (imports, namespaces, definitions, comments, etc.) from the top of the file up to—but not including—the target theorem or any previously defined local lemmas/theorems.
* <target_theorem>...</target_theorem>: The target theorem statement, ending with `:=`.

**Notes**
1. Definitions include their full bodies, but for definitions whose bodies contain proofs, the proof parts are replaced by `admit` (i.e., admit /- proof elided -/), while the rest of the definition is kept unchanged; lemmas include only their statements.
2. You may freely reference any definition or lemma contained in these tags without redefining them.
3. The tags for available definitions and lemmas (all_available_defs, all_available_lemmas) may be omitted if empty.

### Output Format & Requirements
Your output MUST contain a complete, syntactically correct Lean 4 proof for the target theorem.

**Strict Output Structure:**
<reasoning> 
...
</reasoning> 

<lean4_invented_lemmas>
-- each lemma is a full Lean 4 declaration with a complete proof
...
</lean4_invented_lemmas>

<lean4_proof>
-- Lean 4 proof body of the target theorem
...
</lean4_proof>

**Requirements:**
1. <reasoning> and <lean4_invented_lemmas> are OPTIONAL. Include them only if they aid proof construction.
2. <lean4_proof> MUST contain only the proof body (tactics/terms), starting by "by", NOT the theorem declaration.
3. <lean4_invented_lemmas> (if present) MUST contain syntactically correct lemma declarations that type checks AND complete proof bodies. Defining a lemma using `axiom` is NOT allowed.
4. <reasoning> contains high-level proof planning.
5. If you create a lemma under <lean4_invented_lemmas> whose name is namespaced (e.g., X.Y.Z), you MUST always refer to it using its fully qualified name. Do NOT rely on shortened names. For example, write rw [X.Y.Z], not rw [Z] for invented lemmas.
6. All Lean 4 code MUST be syntactically correct. No `sorry`, `admit`, or non-Lean text. Comments allowed using `--` or `/- ... -/`.
7. Follow the tags and structure EXACTLY. No extra text before, between, or after the sections.
8. Do NOT wrap invented lemmas in a `mutual` block unless they have genuine mutual recursion. Declare each lemma independently.

### Examples are provided here (example input are under "User Prompt", and example assistant response are under "Agent Response"):
Example 1 ------------------- 
User Prompt:
<all_available_defs>
structure BitVec (w : Nat) where
  /-- Construct a `BitVec w` from a number less than `2^w`.
  O(1), because we use `Fin` as the internal representation of a bitvector. -/
  ofFin ::

  /-- Interpret a bitvector as a number less than `2^w`.
  O(1), because we use `Fin` as the internal representation of a bitvector. -/
  toFin : Fin (hPow 2 w)
</all_available_defs>

<all_available_lemmas>
theorem List.getElem_append_left{α : Type u_1} {l₁ l₂ : List α} {i : Nat} (hn : i < l₁.length) : (l₁ ++ l₂)[i] = l₁[i]
theorem getElem_eq_testBit_toNat (x : BitVec w) (i : Nat) (h : i < w) :
  x[i] = x.toNat.testBit i := rfl
@[simp, grind =] theorem toNat_cons (b : Bool) (x : BitVec w) :
    (cons b x).toNat = (b.toNat <<< w) ||| x.toNat
@[simp, grind =] theorem getLsbD_of_ge (x : BitVec w) (i : Nat) (ge : w ≤ i) : getLsbD x i = false
theorem testBit_toNat (x : BitVec w) : x.toNat.testBit i = x.getLsbD i
</all_available_lemmas>

<local_ctx>
import Mathlib.Computability.NFA
import Mathlib.Data.FinEnum
import Mathlib.Data.Rel
import Mathlib.Data.Vector.Basic
import Blase.AutoStructs.ForLean

open Set
open Mathlib
open SetRel

-- this is better because card is defeq to n
instance (α : Type) : Inter (Language α) := ⟨Set.inter⟩
instance (α : Type) : Union (Language α) := ⟨Set.union⟩

/--
The set of `n`-tuples of bit vectors of an arbitrary width.
-/
structure BitVecs (n : Nat) where
  w : Nat
  bvs : List.Vector (BitVec w) n

def BitVecs.cast (bvs : BitVecs n) (h : n = n') : BitVecs n' :=
  { w := bvs.w, bvs := h ▸ bvs.bvs }

abbrev BitVecs.empty : BitVecs n := ⟨0, List.Vector.replicate n .nil⟩
abbrev BitVecs.singleton {w : Nat} (bv : BitVec w) : BitVecs 1 := ⟨w, bv ::ᵥ .nil⟩
abbrev BitVecs.pair {w : Nat} (bv1 bv2 : BitVec w) : BitVecs 2 := ⟨w, bv1 ::ᵥ bv2 ::ᵥ .nil⟩

/--
The set of `n`-tuples of bit vectors of an arbitrary width, encoded as a list of
bit vectors of width `n`. The width of the encoded bit vectors is the length of
the list.
-/
abbrev BitVecs' (n : Nat) := List (BitVec n)

@[simps]
def dec (bvs' : BitVecs' n) : BitVecs n where
  w := bvs'.length
  bvs := List.Vector.ofFn fun k => BitVec.ofFn fun i => bvs'[i].getLsbD k

@[simp]
</local_ctx>

<target_theorem>
lemma dec_snoc n (bvs' : BitVecs' n) (a : BitVec n) :
  dec (bvs' ++ [a]) =
    { w := bvs'.length + 1
      bvs := List.Vector.ofFn fun k =>
        BitVec.cons (a.getLsbD k) ((dec bvs').bvs.get k) } :=
</target_theorem>

Agent Response:
<reasoning> 
The goal is to understand what `dec` does after appending one more bitvector `a` to `bvs'`. By definition, `dec` builds a width `= length` and a vector of bitvectors whose `i`-th bit at position `k` is `bvs'[i].getLsbD k`. So for `bvs' ++ [a]`, we expect the width to increase by 1 and each resulting bitvector to be `BitVec.cons` of the new bit coming from `a`.

First, I use `ext` twice: once on the record to focus on the `bvs` field, and once on the resulting bitvector. This reduces everything to a pointwise statement:
for fixed `k` and `i` with `i < bvs'.length + 1`,
the bit `(bvs' ++ [a])[i].getLsbD k` should match the bit at index `i` of `BitVec.cons (a.getLsbD k) ((dec bvs').bvs.get k)`.

At this point I need a clean description of indexing into `BitVec.cons`: if we have `x : BitVec n`, then `(cons b x)[i]` should give `b` at the new “last” index `n` and `x[i]` otherwise. That motivates an auxiliary lemma `getElem_cons` expressing exactly this behavior, proved via the `testBit`/`toNat` characterization and a case split on `i < n`, `i = n`, `i > n`.

Using `getElem_cons`, the RHS becomes an `if` on whether `i = bvs'.length`. So I split into cases:
- If `i = bvs'.length`, then `(bvs' ++ [a])[i]` is the appended `a`, so both sides reduce to `a.getLsbD k`.
- If `i ≠ bvs'.length`, the bound `i < bvs'.length + 1` forces `i < bvs'.length`. Then `(bvs' ++ [a])[i] = bvs'[i]` by `List.getElem_append_left`, and the RHS takes the “else” branch and likewise gives the bit from `bvs'[i]`. In both subcases, `simp` (plus the definition of `dec`) finishes.

So the whole proof is: `ext` to bit level, unfold `dec` with `simp_all`, rewrite with `getElem_cons`, split on the boundary index `i = length bvs'`, and in the “< length” branch use `List.getElem_append_left` and bounds to line up both sides.
</reasoning> 

<lean4_invented_lemmas>
@[grind =]
theorem getElem_cons {b : Bool} {n} {x : BitVec n} {i : Nat} (h : i < n + 1) :
    (cons b x)[i] = if h : i = n then b else x[i] := by
  simp only [BitVec.getElem_eq_testBit_toNat, BitVec.toNat_cons, Nat.testBit_or]
  rw [Nat.testBit_shiftLeft]
  rcases Nat.lt_trichotomy i n with i_lt_n | i_eq_n | n_lt_i
  · have p1 : ¬(n ≤ i) := by omega
    have p2 : i ≠ n := by omega
    simp [p1, p2]
  · simp only [i_eq_n, ge_iff_le, Nat.le_refl, decide_true, Nat.sub_self, Nat.testBit_zero,
    Bool.true_and, BitVec.testBit_toNat, BitVec.getLsbD_of_ge, Bool.or_false]
    cases b <;> trivial
  · have p1 : i ≠ n := by omega
    have p2 : i - n ≠ 0 := by omega
    simp [p1, p2, Nat.testBit_bool_toNat]
</lean4_invented_lemmas>

<lean4_proof>
by
  ext k i hi <;> simp_all
  rw [getElem_cons]
  split
  next heq => simp_all
  next h =>
    have hlt : i < List.length bvs' := by simp at hi; omega
    rw [List.getElem_append_left hlt]
    simp
</lean4_proof>

### Your task:
<all_available_defs>
abbrev MProdWithNames.mk' {α β : Type u} (a : α) (b : β)
  (αName : Lean.Name := default) : MProdWithNames α β αName :=
  @MProdWithNames.mk _ _ αName a b

abbrev WithName.mk' {α : Sort u} (a : α) (name : Lean.Name := default) : WithName α name :=
  a

abbrev WithName.erase {α : Type u} {name} (a : WithName α name) : α := a

private def getDoSeqElems (doSeq : Syntax) : List Syntax :=
  if doSeq.getKind == ``Parser.Term.doSeqBracketed then
    doSeq[1].getArgs.toList.map fun arg => arg[0]
  else if doSeq.getKind == ``Parser.Term.doSeqIndent then
    doSeq[0].getArgs.toList.map fun arg => arg[0]
  else
    []

private def getDoSeq (doStx : Syntax) : Syntax :=
  doStx[1]

private def liftMethodDelimiter (k : SyntaxNodeKind) : Bool :=
  k == ``Parser.Term.do ||
  k == ``Parser.Term.doSeqIndent ||
  k == ``Parser.Term.doSeqBracketed ||
  k == ``Parser.Term.termReturn ||
  k == ``Parser.Term.termUnless ||
  k == ``Parser.Term.termTry ||
  k == ``Parser.Term.termFor

private def letDeclArgHasBinders (letDeclArg : Syntax) : Bool :=
  let k := letDeclArg.getKind
  if k == ``Parser.Term.letPatDecl then
    false
  else if k == ``Parser.Term.letEqnsDecl then
    true
  else if k == ``Parser.Term.letIdDecl then
    
    let binders := letDeclArg[1]
    binders.getNumArgs > 0
  else
    false

private def letDeclHasBinders (letDecl : Syntax) : Bool :=
  letDeclArgHasBinders letDecl[0]

private def liftMethodForbiddenBinder (stx : Syntax) : Bool :=
  let k := stx.getKind
  
  if k == ``Parser.Term.fun || k == ``Parser.Term.matchAlts ||
     k == ``Parser.Term.doLetRec || k == ``Parser.Term.letrec then
     
    true
  
  else if k == ``Parser.Term.let then
    letDeclHasBinders stx[1]
  else if k == ``Parser.Term.doLet then
    letDeclHasBinders stx[2]
  else if k == ``Parser.Term.doLetArrow then
    letDeclArgHasBinders stx[2]
  else
    false

private partial def hasLiftMethod : Syntax → Bool
  | Syntax.node _ k args =>
    if liftMethodDelimiter k then false
    
    
    else if k == ``Parser.Term.liftMethod then true
    
    else if k == ``termDepIfThenElse || k == ``termIfThenElse then args.size >= 2 && hasLiftMethod args[1]!
    else args.any hasLiftMethod
  | _ => false

abbrev Var := Syntax

structure Alt (σ : Type) where
  ref      : Syntax
  vars     : Array Var
  patterns : Syntax
  rhs      : σ
  deriving Inhabited

structure AltExpr (σ : Type) where
  ref     : Syntax
  var?    : Option Var
  funName : Syntax
  pvars   : Array Syntax
  rhs     : σ
  deriving Inhabited

def AltExpr.vars (alt : AltExpr σ) : Array Var := Id.run do
  let mut vars := #[]
  if let some var := alt.var? then
    vars := vars.push var
  for pvar in alt.pvars do
    match pvar with
    | `(_) => pure ()
    | _ => vars := vars.push pvar
  return vars

inductive Code where
  | decl         (xs : Array Var) (doElem : Syntax) (k : Code)
  | reassign     (xs : Array Var) (doElem : Syntax) (k : Code)
   
  | joinpoint    (name : Name) (params : Array (Var × Bool)) (body : Code) (k : Code)
  | seq          (action : Syntax) (k : Code)
  | action       (action : Syntax)
  | break        (ref : Syntax)
  | continue     (ref : Syntax)
  | return       (ref : Syntax) (val : Syntax)
   
  | ite          (ref : Syntax) (h? : Option Var) (optIdent : Syntax) (cond : Syntax) (thenBranch : Code) (elseBranch : Code)
  | match        (ref : Syntax) (gen : Syntax) (discrs : Syntax) (optMotive : Syntax) (alts : Array (Alt Code))
  | matchExpr    (ref : Syntax) («meta» : Bool) (discr : Syntax) (alts : Array (AltExpr Code)) (elseBranch : Code)
  | jmp          (ref : Syntax) (jpName : Name) (args : Array Syntax)
  deriving Inhabited

def Code.getRef? : Code → Option Syntax
  | .decl _ doElem _     => doElem
  | .reassign _ doElem _ => doElem
  | .joinpoint ..        => none
  | .seq a _             => a
  | .action a            => a
  | .break ref           => ref
  | .continue ref        => ref
  | .return ref _        => ref
  | .ite ref ..          => ref
  | .match ref ..        => ref
  | .matchExpr ref ..    => ref
  | .jmp ref ..          => ref

abbrev VarSet := Std.TreeMap Name Syntax Name.cmp

structure CodeBlock where
  code  : Code
  uvars : VarSet := {}

private def varSetToArray (s : VarSet) : Array Var :=
  s.foldl (fun xs _ x => xs.push x) #[]

partial def hasExitPointPred (c : Code) (p : Code → Bool) : Bool :=
  let rec loop : Code → Bool
    | .decl _ _ k             => loop k
    | .reassign _ _ k         => loop k
    | .joinpoint _ _ b k      => loop b || loop k
    | .seq _ k                => loop k
    | .ite _ _ _ _ t e        => loop t || loop e
    | .match _ _ _ _ alts     => alts.any (loop ·.rhs)
    | .matchExpr _ _ _ alts e => alts.any (loop ·.rhs) || loop e
    | .jmp ..                 => false
    | c                       => p c
  loop c

def hasExitPoint (c : Code) : Bool :=
  hasExitPointPred c fun _ => true

def hasReturn (c : Code) : Bool :=
  hasExitPointPred c fun
    | .return .. => true
    | _ => false

def hasTerminalAction (c : Code) : Bool :=
  hasExitPointPred c fun
    | .action _ => true
    | _ => false

def hasBreakContinue (c : Code) : Bool :=
  hasExitPointPred c fun
    | .break _    => true
    | .continue _ => true
    | _ => false

def hasBreakContinueReturn (c : Code) : Bool :=
  hasExitPointPred c fun
    | .break _    => true
    | .continue _ => true
    | .return _ _ => true
    | _ => false

def mkAuxDeclFor {m} [Monad m] [MonadQuotation m] (e : Syntax) (mkCont : Syntax → m Code) : m Code := withRef e <| withFreshMacroScope do
  let y ← `(y)
  let doElem ← `(doElem| let y ← $e:term)
  
  let y ← `(ensure_expected_type% "type mismatch, result value" $y)
  let k ← mkCont y
  return .decl #[y] doElem k

partial def convertTerminalActionIntoJmp (code : Code) (jp : Name) (xs : Array Var) : MacroM Code :=
  let rec loop : Code → MacroM Code
    | .decl xs stx k         => return .decl xs stx (← loop k)
    | .reassign xs stx k     => return .reassign xs stx (← loop k)
    | .joinpoint n ps b k    => return .joinpoint n ps (← loop b) (← loop k)
    | .seq e k               => return .seq e (← loop k)
    | .ite ref x? h c t e    => return .ite ref x? h c (← loop t) (← loop e)
    | .action e              => mkAuxDeclFor e fun y =>
      let ref := e
      
      let jmpArgs := xs.push y
      return Code.jmp ref jp jmpArgs
    | .match ref g ds t alts =>
      return .match ref g ds t (← alts.mapM fun alt => do pure { alt with rhs := (← loop alt.rhs) })
    | .matchExpr ref «meta» d alts e => do
      let alts ← alts.mapM fun alt => do pure { alt with rhs := (← loop alt.rhs) }
      let e ← loop e
      return .matchExpr ref «meta» d alts e
    | c => return c
  loop code

structure JPDecl where
  name : Name
  params : Array (Var × Bool)
  body : Code

def attachJP (jpDecl : JPDecl) (k : Code) : Code :=
  Code.joinpoint jpDecl.name jpDecl.params jpDecl.body k

def attachJPs (jpDecls : Array JPDecl) (k : Code) : Code :=
  jpDecls.foldr attachJP k

def mkFreshJP (ps : Array (Var × Bool)) (body : Code) : TermElabM JPDecl := do
  let ps ← if ps.isEmpty then
    let y ← `(y)
    pure #[(y.raw, false)]
  else
    pure ps
  
  
  
  let name ← mkFreshUserName `__do_jp
  pure { name := name, params := ps, body := body }

def addFreshJP (ps : Array (Var × Bool)) (body : Code) : StateRefT (Array JPDecl) TermElabM Name := do
  let jp ← mkFreshJP ps body
  modify fun (jps : Array JPDecl) => jps.push jp
  pure jp.name

def insertVars (rs : VarSet) (xs : Array Var) : VarSet :=
  xs.foldl (fun rs x => rs.insert x.getId x) rs

def eraseVars (rs : VarSet) (xs : Array Var) : VarSet :=
  xs.foldl (·.erase ·.getId) rs

def eraseOptVar (rs : VarSet) (x? : Option Var) : VarSet :=
  match x? with
  | none   => rs
  | some x => rs.insert x.getId x

def mkSimpleJmp (ref : Syntax) (rs : VarSet) (c : Code) : StateRefT (Array JPDecl) TermElabM Code := do
  let xs := varSetToArray rs
  let jp ← addFreshJP (xs.map fun x => (x, true)) c
  if xs.isEmpty then
    let unit ← ``(Unit.unit)
    return Code.jmp ref jp #[unit]
  else
    return Code.jmp ref jp xs

def mkJmp (ref : Syntax) (rs : VarSet) (val : Syntax) (mkJPBody : Syntax → MacroM Code) : StateRefT (Array JPDecl) TermElabM Code := do
  let xs := varSetToArray rs
  let args := xs.push val
  let yFresh ← withRef ref `(y)
  let ps := xs.map fun x => (x, true)
  let ps := ps.push (yFresh, false)
  let jpBody ← liftMacroM <| mkJPBody yFresh
  let jp ← addFreshJP ps jpBody
  return Code.jmp ref jp args

partial def pullExitPointsAux (rs : VarSet) (c : Code) : StateRefT (Array JPDecl) TermElabM Code := do
  match c with
  | .decl xs stx k         => return .decl xs stx (← pullExitPointsAux (eraseVars rs xs) k)
  | .reassign xs stx k     => return .reassign xs stx (← pullExitPointsAux (insertVars rs xs) k)
  | .joinpoint j ps b k    => return .joinpoint j ps (← pullExitPointsAux rs b) (← pullExitPointsAux rs k)
  | .seq e k               => return .seq e (← pullExitPointsAux rs k)
  | .ite ref x? o c t e    => return .ite ref x? o c (← pullExitPointsAux (eraseOptVar rs x?) t) (← pullExitPointsAux (eraseOptVar rs x?) e)
  | .jmp ..                => return  c
  | .break ref             => mkSimpleJmp ref rs (.break ref)
  | .continue ref          => mkSimpleJmp ref rs (.continue ref)
  | .return ref val        => mkJmp ref rs val (fun y => return .return ref y)
  | .action e              =>
    
    mkAuxDeclFor e fun y =>
      let ref := e
      mkJmp ref rs y (fun yFresh => return .action (← ``(Pure.pure $yFresh)))
  | .match ref g ds t alts =>
    let alts ← alts.mapM fun alt => do pure { alt with rhs := (← pullExitPointsAux (eraseVars rs alt.vars) alt.rhs) }
    return .match ref g ds t alts
  | .matchExpr ref «meta» d alts e =>
    let alts ← alts.mapM fun alt => do pure { alt with rhs := (← pullExitPointsAux (eraseVars rs alt.vars) alt.rhs) }
    let e ← pullExitPointsAux rs e
    return .matchExpr ref «meta» d alts e

def pullExitPoints (c : Code) : TermElabM Code := do
  if hasExitPoint c then
    let (c, jpDecls) ← (pullExitPointsAux {} c).run #[]
    return attachJPs jpDecls c
  else
    return c

partial def extendUpdatedVarsAux (c : Code) (ws : VarSet) : TermElabM Code :=
  let rec update (c : Code) : TermElabM Code := do
    match c with
    | .joinpoint j ps b k    => return .joinpoint j ps (← update b) (← update k)
    | .seq e k               => return .seq e (← update k)
    | .match ref g ds t alts =>
      if alts.any fun alt => alt.vars.any fun x => ws.contains x.getId then
        
        pullExitPoints c
      else
        return .match ref g ds t (← alts.mapM fun alt => do pure { alt with rhs := (← update alt.rhs) })
    | .matchExpr ref «meta» d alts e =>
      if alts.any fun alt => alt.vars.any fun x => ws.contains x.getId then
        
        pullExitPoints c
      else
        let alts ← alts.mapM fun alt => do pure { alt with rhs := (← update alt.rhs) }
        let e ← update e
        return .matchExpr ref «meta» d alts e
    | .ite ref none o c t e => return .ite ref none o c (← update t) (← update e)
    | .ite ref (some h) o cond t e =>
      if ws.contains h.getId then
        
        pullExitPoints c
      else
        return Code.ite ref (some h) o cond (← update t) (← update e)
    | .reassign xs stx k => return .reassign xs stx (← update k)
    | .decl xs stx k => do
      if xs.any fun x => ws.contains x.getId then
        
        pullExitPoints c
      else
        return .decl xs stx (← update k)
    | c => return  c
  update c

partial def extendUpdatedVars (c : CodeBlock) (ws : VarSet) : TermElabM CodeBlock := do
  if ws.any fun x _ => !c.uvars.contains x then
    
    pure { code := (← extendUpdatedVarsAux c.code ws), uvars := ws }
  else
    pure { c with uvars := ws }

private def union (s₁ s₂ : VarSet) : VarSet :=
  s₁.foldl (·.insert ·) s₂

def homogenize (c₁ c₂ : CodeBlock) : TermElabM (CodeBlock × CodeBlock) := do
  let ws := union c₁.uvars c₂.uvars
  let c₁ ← extendUpdatedVars c₁ ws
  let c₂ ← extendUpdatedVars c₂ ws
  pure (c₁, c₂)

def mkVarDeclCore (xs : Array Var) (stx : Syntax) (c : CodeBlock) : CodeBlock := {
  code := Code.decl xs stx c.code,
  uvars := eraseVars c.uvars xs
}

def mkReassignCore (xs : Array Var) (stx : Syntax) (c : CodeBlock) : TermElabM CodeBlock := do
  let us := c.uvars
  let ws := insertVars us xs
  
  
  let code ← if xs.any fun x => !us.contains x.getId then extendUpdatedVarsAux c.code ws else pure c.code
  pure { code := .reassign xs stx code, uvars := ws }

def mkSeq (action : Syntax) (c : CodeBlock) : CodeBlock :=
  { c with code := .seq action c.code }

def mkTerminalAction (action : Syntax) : CodeBlock :=
  { code := .action action }

def mkReturn (ref : Syntax) (val : Syntax) : CodeBlock :=
  { code := .return ref val }

def mkBreak (ref : Syntax) : CodeBlock :=
  { code := .break ref }

def mkContinue (ref : Syntax) : CodeBlock :=
  { code := .continue ref }

def mkIte (ref : Syntax) (optIdent : Syntax) (cond : Syntax) (thenBranch : CodeBlock) (elseBranch : CodeBlock) : TermElabM CodeBlock := do
  let x? := optIdent.getOptional?
  let (thenBranch, elseBranch) ← homogenize thenBranch elseBranch
  return {
    code  := .ite ref x? optIdent cond thenBranch.code elseBranch.code,
    uvars := thenBranch.uvars,
  }

private def mkUnit : MacroM Syntax :=
  ``((⟨⟩ : PUnit))

private def mkPureUnit : MacroM Syntax :=
  ``(pure PUnit.unit)

def mkPureUnitAction : MacroM CodeBlock := do
  return mkTerminalAction (← mkPureUnit)

def mkUnless (cond : Syntax) (c : CodeBlock) : MacroM CodeBlock := do
  let thenBranch ← mkPureUnitAction
  return { c with code := .ite (← getRef) none mkNullNode cond thenBranch.code c.code }

def mkMatch (ref : Syntax) (genParam : Syntax) (discrs : Syntax) (optMotive : Syntax) (alts : Array (Alt CodeBlock)) : TermElabM CodeBlock := do
  
  let ws := alts.foldl (union · ·.rhs.uvars) {}
  let alts ← alts.mapM fun alt => do
    let rhs ← extendUpdatedVars alt.rhs ws
    return { ref := alt.ref, vars := alt.vars, patterns := alt.patterns, rhs := rhs.code : Alt Code }
  return { code := .match ref genParam discrs optMotive alts, uvars := ws }

def mkMatchExpr (ref : Syntax) («meta» : Bool) (discr : Syntax) (alts : Array (AltExpr CodeBlock)) (elseBranch : CodeBlock) : TermElabM CodeBlock := do
  
  let ws := alts.foldl (union · ·.rhs.uvars) {}
  let ws := union ws elseBranch.uvars
  let alts ← alts.mapM fun alt => do
    let rhs ← extendUpdatedVars alt.rhs ws
    return { alt with rhs := rhs.code : AltExpr Code }
  let elseBranch ← extendUpdatedVars elseBranch ws
  return { code := .matchExpr ref «meta» discr alts elseBranch.code, uvars := ws }

def concat (terminal : CodeBlock) (kRef : Syntax) (y? : Option Var) (k : CodeBlock) : TermElabM CodeBlock := do
  unless hasTerminalAction terminal.code do
    throwErrorAt kRef "`do` element is unreachable"
  let (terminal, k) ← homogenize terminal k
  let xs := varSetToArray k.uvars
  let y ← match y? with | some y => pure y | none => `(y)
  let ps := xs.map fun x => (x, true)
  let ps := ps.push (y, false)
  let jpDecl ← mkFreshJP ps k.code
  let jp := jpDecl.name
  let terminal ← liftMacroM <| convertTerminalActionIntoJmp terminal.code jp xs
  return { code  := attachJP jpDecl terminal, uvars := k.uvars }

def getLetIdVars (letId : Syntax) : Array Var :=
  assert! letId.isOfKind ``Parser.Term.letId
  
  if letId[0].isIdent then
    #[letId[0]]
  else if letId[0].isOfKind hygieneInfoKind then
    #[HygieneInfo.mkIdent letId[0] `this (canonical := true)]
  else
    #[]

def getLetIdDeclVars (letIdDecl : Syntax) : Array Var :=
  assert! letIdDecl.isOfKind ``Parser.Term.letIdDecl
  
  
  getLetIdVars letIdDecl[0]

def getPatternVarsEx (pattern : Syntax) : TermElabM (Array Var) :=
  getPatternVars pattern <|>
  Quotation.getPatternVars pattern

def getPatternsVarsEx (patterns : Array Syntax) : TermElabM (Array Var) :=
  getPatternsVars patterns <|>
  Quotation.getPatternsVars patterns

def getLetPatDeclVars (letPatDecl : Syntax) : TermElabM (Array Var) := do
  
  let pattern := letPatDecl[0]
  getPatternVarsEx pattern

def getLetEqnsDeclVars (letEqnsDecl : Syntax) : Array Var :=
  assert! letEqnsDecl.isOfKind ``Parser.Term.letEqnsDecl
  
  
  getLetIdVars letEqnsDecl[0]

def getLetDeclVars (letDecl : Syntax) : TermElabM (Array Var) := do
  
  let arg := letDecl[0]
  if arg.getKind == ``Parser.Term.letIdDecl then
    return getLetIdDeclVars arg
  else if arg.getKind == ``Parser.Term.letPatDecl then
    getLetPatDeclVars arg
  else if arg.getKind == ``Parser.Term.letEqnsDecl then
    return getLetEqnsDeclVars arg
  else
    throwError "unexpected kind of let declaration"

def getDoLetVars (doLet : Syntax) : TermElabM (Array Var) :=
  
  getLetDeclVars doLet[2]

def getDoHaveVars (doHave : Syntax) : TermElabM (Array Var) :=
  
  getLetDeclVars doHave[1]

def getDoLetRecVars (doLetRec : Syntax) : TermElabM (Array Var) := do
  
  let letRecDecls := doLetRec[1][0].getSepArgs
  let letDecls := letRecDecls.map fun p => p[2]
  let mut allVars := #[]
  for letDecl in letDecls do
    let vars ← getLetDeclVars letDecl
    allVars := allVars ++ vars
  return allVars

def getDoReassignVars (doReassign : Syntax) : TermElabM (Array Var) := do
  let arg := doReassign[0]
  if arg.getKind == ``Parser.Term.letIdDecl then
    return getLetIdDeclVars arg
  else if arg.getKind == ``Parser.Term.letPatDecl then
    getLetPatDeclVars arg
  else
    throwError "unexpected kind of reassignment"

def mkDoSeq (doElems : Array Syntax) : Syntax :=
  mkNode `Lean.Parser.Term.doSeqIndent #[mkNullNode <| doElems.map fun doElem => mkNullNode #[doElem, mkNullNode]]

structure DoIfView where
  ref        : Syntax
  optIdent   : Syntax
  cond       : Syntax
  thenBranch : Syntax
  elseBranch : Syntax

private def mkDoIfView (doIf : Syntax) : DoIfView := {
  ref        := doIf
  optIdent   := doIf[1][0]
  cond       := doIf[1][1]
  thenBranch := doIf[3]
  elseBranch := doIf[5][1]
}

private def mkTuple (elems : Array Syntax) : MacroM Syntax := do
  if elems.size = 0 then
    mkUnit
  else if h : elems.size = 1 then
    ``(WithName.mk' $(elems[0]) $(Lean.quoteNameMk elems[0].getId))
  else
    let init <- ``(WithName.mk' $(elems.back!) $(Lean.quoteNameMk elems.back!.getId))
    elems.extract 0 (elems.size - 1) |>.foldrM (init := init) fun elem tuple => do
      let name := Lean.quoteNameMk elem.getId
      ``(MProdWithNames.mk' $elem $tuple $name)

private def mkTuplePat (elems : Array Syntax) : MacroM Syntax := do
  if elems.size = 0 then
    mkUnit
  else if h : elems.size = 1 then
    return elems[0]
  else
    elems.extract 0 (elems.size - 1) |>.foldrM (init := elems.back!) fun elem tuple => do
        ``(MProdWithNames.mk $elem $tuple)

def isDoExpr? (doElem : Syntax) : Option Syntax :=
  if doElem.getKind == ``Parser.Term.doExpr then
    some doElem[0]
  else
    none

private def destructTuple (uvars : Array Var) (x : Syntax) (body : Syntax) : MacroM Syntax := do
  if uvars.size = 0 then
    return body
  else if h : uvars.size = 1 then
    `(let $(uvars[0]):ident := WithName.erase $x; $body)
  else
    destruct uvars.toList x body
where
  destruct (as : List Var) (x : Syntax) (body : Syntax) : MacroM Syntax := do
    match as with
      | [a, b]  => `(let $a:ident := $x.1; let $b:ident := WithName.erase $x.2; $body)
      | a :: as => withFreshMacroScope do
        let rest ← destruct as (← `(x)) body
        `(let $a:ident := $x.1; let x := $x.2; $rest)
      | _ => unreachable!

def annotate [Monad m] [MonadRef m] [MonadQuotation m] (ref : Syntax) (term : Syntax) : m Syntax :=
  withRef term <| `(with_annotate_term $ref $term)

inductive Kind where
  | regular
  | forIn
  | forInWithReturn
  | nestedBC
  | nestedPR
  | nestedSBC
  | nestedPRBC

structure Context where
   
  m          : Syntax
   
  returnType : Syntax
  uvars      : Array Var
  kind       : Kind

abbrev M := ReaderT Context MacroM

def mkUVarTuple : M Syntax := do
  let ctx ← read
  mkTuple ctx.uvars

def returnToTerm (val : Syntax) : M Syntax := do
  let ctx ← read
  let u ← mkUVarTuple
  match ctx.kind with
  | .regular         => if ctx.uvars.isEmpty then ``(Pure.pure $val) else ``(Pure.pure (MProdWithNames.mk $val $u))
  | .forIn           => ``(Pure.pure (ForInStep.done $u))
  | .forInWithReturn => ``(Pure.pure (ForInStep.done (MProdWithNames.mk (some $val) $u)))
  | .nestedBC        => unreachable!
  | .nestedPR        => ``(Pure.pure (DoResultPR.«return» $val $u))
  | .nestedSBC       => ``(Pure.pure (DoResultSBC.«pureReturn» $val $u))
  | .nestedPRBC      => ``(Pure.pure (DoResultPRBC.«return» $val $u))

def continueToTerm : M Syntax := do
  let ctx ← read
  let u ← mkUVarTuple
  match ctx.kind with
  | .regular         => unreachable!
  | .forIn           => ``(Pure.pure (ForInStep.yield $u))
  | .forInWithReturn => ``(Pure.pure (ForInStep.yield (MProdWithNames.mk none $u)))
  | .nestedBC        => ``(Pure.pure (DoResultBC.«continue» $u))
  | .nestedPR        => unreachable!
  | .nestedSBC       => ``(Pure.pure (DoResultSBC.«continue» $u))
  | .nestedPRBC      => ``(Pure.pure (DoResultPRBC.«continue» $u))

def breakToTerm : M Syntax := do
  let ctx ← read
  let u ← mkUVarTuple
  match ctx.kind with
  | .regular         => unreachable!
  | .forIn           => ``(Pure.pure (ForInStep.done $u))
  | .forInWithReturn => ``(Pure.pure (ForInStep.done (MProdWithNames.mk none $u)))
  | .nestedBC        => ``(Pure.pure (DoResultBC.«break» $u))
  | .nestedPR        => unreachable!
  | .nestedSBC       => ``(Pure.pure (DoResultSBC.«break» $u))
  | .nestedPRBC      => ``(Pure.pure (DoResultPRBC.«break» $u))

def actionTerminalToTerm (action : Syntax) : M Syntax := withRef action <| withFreshMacroScope do
  let ctx ← read
  let u ← mkUVarTuple
  match ctx.kind with
  | .regular         => if ctx.uvars.isEmpty then pure action else ``(Bind.bind $action fun y => Pure.pure (MProdWithNames.mk y $u))
  | .forIn           => ``(Bind.bind $action fun (_ : PUnit) => Pure.pure (ForInStep.yield $u))
  | .forInWithReturn => ``(Bind.bind $action fun (_ : PUnit) => Pure.pure (ForInStep.yield (MProdWithNames.mk none $u)))
  | .nestedBC        => unreachable!
  | .nestedPR        => ``(Bind.bind $action fun y => (Pure.pure (DoResultPR.«pure» y $u)))
  | .nestedSBC       => ``(Bind.bind $action fun y => (Pure.pure (DoResultSBC.«pureReturn» y $u)))
  | .nestedPRBC      => ``(Bind.bind $action fun y => (Pure.pure (DoResultPRBC.«pure» y $u)))

def seqToTerm (action : Syntax) (k : Syntax) : M Syntax := withRef action <| withFreshMacroScope do
  if action.getKind == ``Parser.Term.doDbgTrace then
    let msg := action[1]
    `(dbg_trace $msg; $k)
  else if action.getKind == ``Parser.Term.doAssert then
    let cond := action[1]
    `(assert! $cond; $k)
  else if action.getKind == ``Parser.Term.doDebugAssert then
    let cond := action[1]
    `(debugAssert| debug_assert! $cond; $k)
  else
    let action ← withRef action ``(($action : $((←read).m) PUnit))
    ``(Bind.bind $action (fun (_ : PUnit) => $k))

def declToTerm (decl : Syntax) (k : Syntax) : M Syntax := withRef decl <| withFreshMacroScope do
  let kind := decl.getKind
  if kind == ``Parser.Term.doLet then
    let letDecl := decl[2]
    `(let $letDecl:letDecl; $k)
  else if kind == ``Parser.Term.doLetRec then
    let letRecToken := decl[0]
    let letRecDecls := decl[1]
    return mkNode ``Parser.Term.letrec #[letRecToken, letRecDecls, mkNullNode, k]
  else if kind == ``Parser.Term.doLetArrow then
    let arg := decl[2]
    if arg.getKind == ``Parser.Term.doIdDecl then
      let id     := arg[0]
      let type   := expandOptType id arg[1]
      let doElem := arg[3]
      
      match isDoExpr? doElem with
      | some action =>
        let action ← withRef action `(($action : $((← read).m) $type))
        ``(Bind.bind $action (fun ($id:ident : $type) => $k))
      | none        => Macro.throwErrorAt decl "unexpected kind of `do` declaration"
    else
      Macro.throwErrorAt decl "unexpected kind of `do` declaration"
  else if kind == ``Parser.Term.doHave then
    
    let args := decl.getArgs
    let args := args ++ #[mkNullNode  , k]
    return mkNode `Lean.Parser.Term.«have» args
  else
    Macro.throwErrorAt decl "unexpected kind of `do` declaration"

def reassignToTerm (reassign : Syntax) (k : Syntax) : MacroM Syntax := withRef reassign <| withFreshMacroScope do
  match reassign with
  | `(doElem| $x:ident := $rhs) => `(let $x:ident := ensure_type_of% $x $(quote "invalid reassignment, value") $rhs; $k)
  | `(doElem| $e:term  := $rhs) => `(let $e:term  := ensure_type_of% $e $(quote "invalid reassignment, value") $rhs; $k)
  | _ =>
    
    Macro.throwErrorAt reassign "unexpected kind of `do` reassignment"

def mkIte (optIdent : Syntax) (cond : Syntax) (thenBranch : Syntax) (elseBranch : Syntax) : MacroM Syntax := do
  if optIdent.isNone then
    ``(if $cond then $thenBranch else $elseBranch)
  else
    let h := optIdent[0]
    ``(if $h:ident : $cond then $thenBranch else $elseBranch)

def mkJoinPoint (j : Name) (ps : Array (Syntax × Bool)) (body : Syntax) (k : Syntax) : M Syntax := withRef body <| withFreshMacroScope do
  let pTypes ← ps.mapM fun ⟨id, useTypeOf⟩ => do if useTypeOf then `(type_of% $id) else `(_)
  let ps     := ps.map (·.1)
   
  `(let_delayed $(← mkIdentFromRef j):ident $[($ps : $pTypes)]* : $((← read).m) _ := $body; $k)

def mkJmp (ref : Syntax) (j : Name) (args : Array Syntax) : Syntax :=
  Syntax.mkApp (mkIdentFrom ref j) args

partial def toTerm (c : Code) : M Syntax := do
  let term ← go c
  if let some ref := c.getRef? then
    annotate ref term
  else
    return term
where
  go (c : Code) : M Syntax := do
    match c with
    | .return ref val     => withRef ref <| returnToTerm val
    | .continue ref       => withRef ref continueToTerm
    | .break ref          => withRef ref breakToTerm
    | .action e           => actionTerminalToTerm e
    | .joinpoint j ps b k => mkJoinPoint j ps (← toTerm b) (← toTerm k)
    | .jmp ref j args     => return mkJmp ref j args
    | .decl _ stx k       => declToTerm stx (← toTerm k)
    | .reassign _ stx k   => reassignToTerm stx (← toTerm k)
    | .seq stx k          => seqToTerm stx (← toTerm k)
    | .ite ref _ o c t e  => withRef ref <| do mkIte o c (← toTerm t) (← toTerm e)
    | .match ref genParam discrs optMotive alts =>
      let mut termAlts := #[]
      for alt in alts do
        let rhs ← toTerm alt.rhs
        let termAlt := mkNode ``Parser.Term.matchAlt #[mkAtomFrom alt.ref "|", mkNullNode #[alt.patterns], mkAtomFrom alt.ref "=>", rhs]
        termAlts := termAlts.push termAlt
      let termMatchAlts := mkNode ``Parser.Term.matchAlts #[mkNullNode termAlts]
      return mkNode ``Parser.Term.«match» #[mkAtomFrom ref "match", genParam, optMotive, discrs, mkAtomFrom ref "with", termMatchAlts]
    | .matchExpr ref «meta» d alts elseBranch => withFreshMacroScope do
      let d' ← `(discr)
      let mut termAlts := #[]
      for alt in alts do
        let rhs ← `(($(← toTerm alt.rhs) : $((← read).m) _))
        let optVar := if let some var := alt.var? then mkNullNode #[var, mkAtomFrom var "@"] else mkNullNode #[]
        let pat := mkNode ``Parser.Term.matchExprPat #[optVar, alt.funName, mkNullNode alt.pvars]
        let termAlt := mkNode ``Parser.Term.matchExprAlt #[mkAtomFrom alt.ref "|", pat, mkAtomFrom alt.ref "=>", rhs]
        termAlts := termAlts.push termAlt
      let elseBranch := mkNode ``Parser.Term.matchExprElseAlt #[mkAtomFrom ref "|", mkHole ref, mkAtomFrom ref "=>", (← toTerm elseBranch)]
      let termMatchExprAlts := mkNode ``Parser.Term.matchExprAlts #[mkNullNode termAlts, elseBranch]
      let body := mkNode ``Parser.Term.matchExpr #[mkAtomFrom ref "match_expr", d', mkAtomFrom ref "with", termMatchExprAlts]
      if «meta» then
        `(Bind.bind (instantiateMVarsIfMVarApp $d) fun discr => $body)
      else
        `(let discr := $d; $body)

def run (code : Code) (m : Syntax) (returnType : Syntax) (uvars : Array Var := #[]) (kind := Kind.regular) : MacroM Syntax :=
  toTerm code { m, returnType, kind, uvars }

def mkNestedKind (a r bc : Bool) : Kind :=
  match a, r, bc with
  | true,  false, false => .regular
  | false, true,  false => .regular
  | false, false, true  => .nestedBC
  | true,  true,  false => .nestedPR
  | true,  false, true  => .nestedSBC
  | false, true,  true  => .nestedSBC
  | true,  true,  true  => .nestedPRBC
  | false, false, false => unreachable!

def mkNestedTerm (code : Code) (m : Syntax) (returnType : Syntax) (uvars : Array Var) (a r bc : Bool) : MacroM Syntax := do
  ToTerm.run code m returnType uvars (mkNestedKind a r bc)

def matchNestedTermResult (term : Syntax) (uvars : Array Var) (a r bc : Bool) : MacroM (List Syntax) := do
  let toDoElems (auxDo : Syntax) : List Syntax := getDoSeqElems (getDoSeq auxDo)
  let u ← mkTuplePat uvars
  match a, r, bc with
  | true, false, false =>
    if uvars.isEmpty then
      return toDoElems (← `(do $term:term))
    else
      return toDoElems (← `(do let r ← $term:term; $u:term := r.2; pure r.1))
  | false, true, false =>
    if uvars.isEmpty then
      return toDoElems (← `(do let r ← $term:term; return r))
    else
      return toDoElems (← `(do let r ← $term:term; $u:term := r.2; return r.1))
  | false, false, true => toDoElems <$>
    `(do let r ← $term:term;
         match r with
         | .break u => $u:term := u; break
         | .continue u => $u:term := u; continue)
  | true, true, false => toDoElems <$>
    `(do let r ← $term:term;
         match r with
         | .pure a u => $u:term := u; pure a
         | .return b u => $u:term := u; return b)
  | true, false, true => toDoElems <$>
    `(do let r ← $term:term;
         match r with
         | .pureReturn a u => $u:term := u; pure a
         | .break u => $u:term := u; break
         | .continue u => $u:term := u; continue)
  | false, true, true => toDoElems <$>
    `(do let r ← $term:term;
         match r with
         | .pureReturn a u => $u:term := u; return a
         | .break u => $u:term := u; break
         | .continue u => $u:term := u; continue)
  | true, true, true => toDoElems <$>
    `(do let r ← $term:term;
         match r with
         | .pure a u => $u:term := u; pure a
         | .return a u => $u:term := u; return a
         | .break u => $u:term := u; break
         | .continue u => $u:term := u; continue)
  | false, false, false => unreachable!

def isMutableLet (doElem : Syntax) : Bool :=
  let kind := doElem.getKind
  (kind == ``doLetArrow || kind == ``doLet || kind == ``doLetElse)
  &&
  !doElem[1].isNone

structure Context where
  ref         : Syntax
   
  m           : Syntax
   
  returnType  : Syntax
  mutableVars : VarSet := {}
  insideFor   : Bool := false

abbrev M := ReaderT Context TermElabM

def withNewMutableVars {α} (newVars : Array Var) (mutable : Bool) (x : M α) : M α :=
  withReader (fun ctx => if mutable then { ctx with mutableVars := insertVars ctx.mutableVars newVars } else ctx) x

def checkReassignable (xs : Array Var) : M Unit := do
  let throwInvalidReassignment (x : Name) : M Unit :=
    throwError "`{x.simpMacroScopes}` cannot be mutated, only variables declared using `let mut` can be mutated. If you did not intend to mutate but define `{x.simpMacroScopes}`, consider using `let {x.simpMacroScopes}` instead"
  let ctx ← read
  for x in xs do
    unless ctx.mutableVars.contains x.getId do
      throwInvalidReassignment x.getId

def checkNotShadowingMutable (xs : Array Var) : M Unit := do
  let throwInvalidShadowing (x : Name) : M Unit :=
    throwError "mutable variable `{x.simpMacroScopes}` cannot be shadowed"
  let ctx ← read
  for x in xs do
    if ctx.mutableVars.contains x.getId then
      withRef x <| throwInvalidShadowing x.getId

def withFor {α} (x : M α) : M α :=
  withReader (fun ctx => { ctx with insideFor := true }) x

structure ToForInTermResult where
  uvars      : Array Var
  term       : Syntax

def mkForInBody  (_ : Syntax) (forInBody : CodeBlock) : M ToForInTermResult := do
  let ctx ← read
  let uvars := forInBody.uvars
  let uvars := varSetToArray uvars
  let term ← liftMacroM <| ToTerm.run forInBody.code ctx.m ctx.returnType uvars (if hasReturn forInBody.code then ToTerm.Kind.forInWithReturn else ToTerm.Kind.forIn)
  return ⟨uvars, term⟩

def ensureInsideFor : M Unit :=
  unless (← read).insideFor do
    throwError "invalid `do` element, it must be inside `for`"

def ensureEOS (doElems : List Syntax) : M Unit :=
  unless doElems.isEmpty do
    throwError "must be last element in a `do` sequence"

private partial def expandLiftMethodAux (inQuot : Bool) (inBinder : Bool) : Syntax → StateT (List Syntax) M Syntax
  | stx@(Syntax.node i k args) =>
    if k == choiceKind then do
      
      let alts ← stx.getArgs.mapM (expandLiftMethodAux inQuot inBinder · |>.run [])
      let (_, lifts) := alts[0]!
      unless alts.all (·.2 == lifts) do
        throwErrorAt stx "cannot lift `(<- ...)` over inconsistent syntax variants, consider lifting out the binding manually"
      modify (· ++ lifts)
      return .node i k (alts.map (·.1))
    else if liftMethodDelimiter k then
      return stx
    
    else if h : args.size >= 2 ∧ (k == ``termDepIfThenElse || k == ``termIfThenElse) then do
      let inAntiquot := stx.isAntiquot && !stx.isEscapedAntiquot
      let arg1 ← expandLiftMethodAux (inQuot && !inAntiquot || stx.isQuot) inBinder args[1]
      let args := args.set! 1 arg1
      return Syntax.node i k args
    else if k == ``Parser.Term.liftMethod && !inQuot then withFreshMacroScope do
      if inBinder then
        throwErrorAt stx "cannot lift `(<- ...)` over a binder, this error usually happens when you are trying to lift a method nested in a `fun`, `let`, or `match`-alternative, and it can often be fixed by adding a missing `do`"
      let term := args[1]!
      let term ← expandLiftMethodAux inQuot inBinder term
      
      let id ← mkIdentFromRef (.num baseId (← get).length)
      let auxDoElem : Syntax ← `(doElem| let $id:ident ← $term:term)
      modify fun s => s ++ [auxDoElem]
      return id
    else do
      let inAntiquot := stx.isAntiquot && !stx.isEscapedAntiquot
      let inBinder   := inBinder || (!inQuot && liftMethodForbiddenBinder stx)
      let args ← args.mapM (expandLiftMethodAux (inQuot && !inAntiquot || stx.isQuot) inBinder)
      return Syntax.node i k args
  | stx => return stx

def expandLiftMethod (doElem : Syntax) : M (List Syntax × Syntax) := do
  if !hasLiftMethod doElem then
    return ([], doElem)
  else
    let baseId ← withFreshMacroScope (MonadQuotation.addMacroScope `__do_lift)
    let (doElem, doElemsNew) ← (expandLiftMethodAux baseId false false doElem).run []
    return (doElemsNew, doElem)

def checkLetArrowRHS (doElem : Syntax) : M Unit := do
  let kind := doElem.getKind
  if kind == ``Parser.Term.doLetArrow ||
     kind == ``Parser.Term.doLet ||
     kind == ``Parser.Term.doLetRec ||
     kind == ``Parser.Term.doHave ||
     kind == ``Parser.Term.doReassign ||
     kind == ``Parser.Term.doReassignArrow then
    throwErrorAt doElem "invalid kind of value `{kind}` in an assignment"

def doReturnToCode (doReturn : Syntax) (doElems: List Syntax) : M CodeBlock := withRef doReturn do
  ensureEOS doElems
  let argOpt := doReturn[1]
  let arg ← if argOpt.isNone then liftMacroM mkUnit else pure argOpt[0]
  return mkReturn (← getRef) arg

structure Catch where
  x         : Syntax
  optType   : Syntax
  codeBlock : CodeBlock

def getTryCatchUpdatedVars (tryCode : CodeBlock) (catches : Array Catch) (finallyCode? : Option CodeBlock) : VarSet :=
  let ws := tryCode.uvars
  let ws := catches.foldl (init := ws) fun ws alt => union alt.codeBlock.uvars ws
  let ws := match finallyCode? with
    | none   => ws
    | some c => union c.uvars ws
  ws

def tryCatchPred (tryCode : CodeBlock) (catches : Array Catch) (finallyCode? : Option CodeBlock) (p : Code → Bool) : Bool :=
  p tryCode.code ||
  catches.any (fun «catch» => p «catch».codeBlock.code) ||
  match finallyCode? with
  | none => false
  | some finallyCode => p finallyCode.code

partial def concatWith (c : CodeBlock) (doElems : List Syntax) : M CodeBlock :=
    match doElems with
    | [] => pure c
    | nextDoElem :: _  => do
      let k ← doSeqToCode doElems
      let ref := nextDoElem
      concat c ref none k

partial def doLetArrowToCode (doLetArrow : Syntax) (doElems : List Syntax) : M CodeBlock := do
    let decl    := doLetArrow[2]
    if decl.getKind == ``Parser.Term.doIdDecl then
      let y := decl[0]
      checkNotShadowingMutable #[y]
      let doElem := decl[3]
      let k ← withNewMutableVars #[y] (isMutableLet doLetArrow) (doSeqToCode doElems)
      match isDoExpr? doElem with
      | some _      => return mkVarDeclCore #[y] doLetArrow k
      | none =>
        checkLetArrowRHS doElem
        let c ← doSeqToCode [doElem]
        match doElems with
        | []       => pure c
        | kRef::_  => concat c kRef y k
    else if decl.getKind == ``Parser.Term.doPatDecl then
      let pattern := decl[0]
      let doElem  := decl[2]
      let optElse := decl[3]
      if optElse.isNone then withFreshMacroScope do
        let auxDo ← if isMutableLet doLetArrow then
          `(do let%$doLetArrow __discr ← $doElem; let%$doLetArrow mut $pattern:term := __discr)
        else
          `(do let%$doLetArrow __discr ← $doElem; let%$doLetArrow $pattern:term := __discr)
        doSeqToCode <| getDoSeqElems (getDoSeq auxDo) ++ doElems
      else
        let contSeq ← if isMutableLet doLetArrow then
          let vars ← (← getPatternVarsEx pattern).mapM fun var => `(doElem| let mut $var := $var)
          pure (vars ++ doElems.toArray)
        else
          pure doElems.toArray
        let contSeq := mkDoSeq contSeq
        let elseSeq := optElse[1]
        let auxDo ← `(do let%$doLetArrow __discr ← $doElem; match%$doLetArrow __discr with | $pattern:term => $contSeq | _ => $elseSeq)
        doSeqToCode <| getDoSeqElems (getDoSeq auxDo)
    else
      throwError "unexpected kind of `do` declaration"

partial def doLetElseToCode (doLetElse : Syntax) (doElems : List Syntax) : M CodeBlock := do
    
    let pattern := doLetElse[2]
    let val     := doLetElse[4]
    let elseSeq := doLetElse[6]
    let contSeq ← if isMutableLet doLetElse then
      let vars ← (← getPatternVarsEx pattern).mapM fun var => `(doElem| let mut $var := $var)
      pure (vars ++ doElems.toArray)
    else
      pure doElems.toArray
    let contSeq := mkDoSeq contSeq
    let auxDo ← `(do match $val:term with | $pattern:term => $contSeq | _ => $elseSeq)
    doSeqToCode <| getDoSeqElems (getDoSeq auxDo)

partial def doReassignArrowToCode (doReassignArrow : Syntax) (doElems : List Syntax) : M CodeBlock := do
    let decl := doReassignArrow[0]
    if decl.getKind == ``Parser.Term.doIdDecl then
      let doElem := decl[3]
      let y      := decl[0]
      let auxDo ← `(do let r ← $doElem; $y:ident := r)
      doSeqToCode <| getDoSeqElems (getDoSeq auxDo) ++ doElems
    else if decl.getKind == ``Parser.Term.doPatDecl then
      let pattern := decl[0]
      let doElem  := decl[2]
      let optElse := decl[3]
      if optElse.isNone then withFreshMacroScope do
        let auxDo ← `(do let __discr ← $doElem; $pattern:term := __discr)
        doSeqToCode <| getDoSeqElems (getDoSeq auxDo) ++ doElems
      else
        throwError "reassignment with `|` (i.e., \"else clause\") is not currently supported"
    else
      throwError "unexpected kind of `do` reassignment"

partial def doIfToCode (doIf : Syntax) (doElems : List Syntax) : M CodeBlock := do
    let view := mkDoIfView doIf
    let thenBranch ← doSeqToCode (getDoSeqElems view.thenBranch)
    let elseBranch ← doSeqToCode (getDoSeqElems view.elseBranch)
    let ite ← mkIte view.ref view.optIdent view.cond thenBranch elseBranch
    concatWith ite doElems

partial def doUnlessToCode (doUnless : Syntax) (doElems : List Syntax) : M CodeBlock := withRef doUnless do
    let cond  := doUnless[1]
    let doSeq := doUnless[3]
    let body ← doSeqToCode (getDoSeqElems doSeq)
    let unlessCode ← liftMacroM <| mkUnless cond body
    concatWith unlessCode doElems

partial def doForToCode (doFor : Syntax) (doElems : List Syntax) : M CodeBlock := do
    let doForDecls := doFor[1].getSepArgs
    if h : doForDecls.size > 1 then
       
      
      let doForDecl := doForDecls[1]!
      unless doForDecl[0].isNone do
        throwErrorAt doForDecl[0] "the proof annotation here has not been implemented yet"
      let y  := doForDecl[1]
      let ys := doForDecl[3]
      let doForDecls := doForDecls.eraseIdx 1
      let body := doFor[3]
      withFreshMacroScope do
         
        let toStreamApp ← withRef ys `(@toStream _ _ _ $ys)
        let auxDo ←
          `(do let mut s := $toStreamApp:term
               for $doForDecls:doForDecl,* do
                 match @Stream.next? _ _ _ s with
                 | none => break
                 | some ($y, s') =>
                   s := s'
                   do $body)
        doSeqToCode (getDoSeqElems (getDoSeq auxDo) ++ doElems)
    else withRef doFor do
      let h?        := if doForDecls[0]![0].isNone then none else some doForDecls[0]![0][0]
      let x         := doForDecls[0]![1]
      withRef x <| checkNotShadowingMutable (← getPatternVarsEx x)
      let xs        := doForDecls[0]![3]
      let forElems  := getDoSeqElems doFor[3]
      let forInBodyCodeBlock ← withFor (doSeqToCode forElems)
      let ⟨uvars, forInBody⟩ ← mkForInBody x forInBodyCodeBlock
      let ctx ← read
      
      
      
      let uvars := uvars.map fun v => ctx.mutableVars.getD v.getId v
      let uvarsTuple ← liftMacroM do mkTuple uvars
      let uvarsTuplePat ← liftMacroM do mkTuplePat uvars

if hasReturn forInBodyCodeBlock.code then
        let forInBody ← liftMacroM <| destructTuple uvars (← `(r)) forInBody
        let optType ← `(Option $((← read).returnType))
        let forInTerm ← if let some h := h? then
          annotate doFor
            (← `(for_in'% $(xs) (MProdWithNames.mk (none : $optType) $uvarsTuple) fun $x $h (r : MProdWithNames $optType _) => let r := r.2; $forInBody))
        else
          annotate doFor
            (← `(for_in% $(xs) (MProdWithNames.mk (none : $optType) $uvarsTuple) fun $x (r : MProdWithNames $optType _) => let r := r.2; $forInBody))
        let auxDo ← `(do let r ← $forInTerm:term;
                         $uvarsTuplePat:term := r.2;
                         match r.1 with
                         | none => Pure.pure (ensure_expected_type% "type mismatch, `for`" PUnit.unit)
                         | some a => return ensure_expected_type% "type mismatch, `for`" a)
        doSeqToCode (getDoSeqElems (getDoSeq auxDo) ++ doElems)
      else
        let forInBody ← liftMacroM <| destructTuple uvars (← `(r)) forInBody
        let forInTerm ← if let some h := h? then
          annotate doFor (← `(for_in'% $(xs) $uvarsTuple fun $x $h r => $forInBody))
        else
          annotate doFor (← `(for_in% $(xs) $uvarsTuple fun $x r => $forInBody))
        if doElems.isEmpty then
          let auxDo ← `(do let r ← $forInTerm:term;
                           $uvarsTuplePat:term := r;
                           Pure.pure (ensure_expected_type% "type mismatch, `for`" PUnit.unit))
          doSeqToCode <| getDoSeqElems (getDoSeq auxDo)
        else
          let auxDo ← `(do let r ← $forInTerm:term; $uvarsTuplePat:term := r)
          doSeqToCode <| getDoSeqElems (getDoSeq auxDo) ++ doElems

partial def doMatchToCode (doMatch : Syntax) (doElems: List Syntax) : M CodeBlock := do
    let ref       := doMatch
    let genParam  := doMatch[1]
    let optMotive := doMatch[2]
    let discrs    := doMatch[3]
    let matchAlts := doMatch[5][0].getArgs 
    let matchAlts ← matchAlts.foldlM (init := #[]) fun result matchAlt => return result ++ (← liftMacroM <| expandMatchAlt matchAlt)
    let alts ←  matchAlts.mapM fun matchAlt => do
      let patterns := matchAlt[1][0]
      let vars ← getPatternsVarsEx patterns.getSepArgs
      withRef patterns <| checkNotShadowingMutable vars
      let rhs  := matchAlt[3]
      let rhs ← doSeqToCode (getDoSeqElems rhs)
      pure { ref := matchAlt, vars := vars, patterns := patterns, rhs := rhs : Alt CodeBlock }
    let matchCode ← mkMatch ref genParam discrs optMotive alts
    concatWith matchCode doElems

partial def doMatchExprToCode (doMatchExpr : Syntax) (doElems: List Syntax) : M CodeBlock := do
    let ref       := doMatchExpr
    let «meta»    := doMatchExpr[1].isNone
    let discr     := doMatchExpr[2]
    let alts      := doMatchExpr[4][0].getArgs 
    let alts ← alts.mapM fun alt => do
      let pat      := alt[1]
      let var?     := if pat[0].isNone then none else some pat[0][0]
      let funName  := pat[1]
      let pvars    := pat[2].getArgs
      let rhs      := alt[3]
      let rhs ← doSeqToCode (getDoSeqElems rhs)
      pure { ref, var?, funName, pvars, rhs }
    let elseBranch ← doSeqToCode (getDoSeqElems doMatchExpr[4][1][3])
    let matchCode ← mkMatchExpr ref «meta» discr alts elseBranch
    concatWith matchCode doElems

partial def doTryToCode (doTry : Syntax) (doElems: List Syntax) : M CodeBlock := do
    let tryCode ← doSeqToCode (getDoSeqElems doTry[1])
    let optFinally := doTry[3]
    let catches ← doTry[2].getArgs.mapM fun catchStx : Syntax => do
      if catchStx.getKind == ``Parser.Term.doCatch then
        let x       := catchStx[1]
        if x.isIdent then
          withRef x <| checkNotShadowingMutable #[x]
        let optType := catchStx[2]
        let c ← doSeqToCode (getDoSeqElems catchStx[4])
        return { x := x, optType := optType, codeBlock := c : Catch }
      else if catchStx.getKind == ``Parser.Term.doCatchMatch then
        let matchAlts := catchStx[1]
        let x ← `(ex)
        let auxDo ← `(do match ex with $matchAlts)
        let c ← doSeqToCode (getDoSeqElems (getDoSeq auxDo))
        return { x := x, codeBlock := c, optType := mkNullNode : Catch }
      else
        throwError "unexpected kind of `catch`"
    let finallyCode? ← if optFinally.isNone then pure none else some <$> doSeqToCode (getDoSeqElems optFinally[0][1])
    if catches.isEmpty && finallyCode?.isNone then
      throwError "invalid `try`, it must have a `catch` or `finally`"
    let ctx ← read
    let ws    := getTryCatchUpdatedVars tryCode catches finallyCode?
    let uvars := varSetToArray ws
    let a     := tryCatchPred tryCode catches finallyCode? hasTerminalAction
    let r     := tryCatchPred tryCode catches finallyCode? hasReturn
    let bc    := tryCatchPred tryCode catches finallyCode? hasBreakContinue
    let toTerm (codeBlock : CodeBlock) : M Syntax := do
      let codeBlock ← liftM $ extendUpdatedVars codeBlock ws
      liftMacroM <| ToTerm.mkNestedTerm codeBlock.code ctx.m ctx.returnType uvars a r bc
    let term ← toTerm tryCode
    let term ← catches.foldlM (init := term) fun term «catch» => do
      let catchTerm ← toTerm «catch».codeBlock
      if catch.optType.isNone then
        annotate doTry (← ``(MonadExcept.tryCatch $term (fun $(«catch».x):ident => $catchTerm)))
      else
        let type := «catch».optType[1]
        annotate doTry (← ``(tryCatchThe $type $term (fun $(«catch».x):ident => $catchTerm)))
    let term ← match finallyCode? with
      | none             => pure term
      | some finallyCode => withRef optFinally do
        unless finallyCode.uvars.isEmpty do
          throwError "`finally` currently does not support reassignments"
        if hasBreakContinueReturn finallyCode.code then
          throwError "`finally` currently does `return`, `break`, nor `continue`"
        let finallyTerm ← liftMacroM <| ToTerm.run finallyCode.code ctx.m ctx.returnType {} ToTerm.Kind.regular
        annotate doTry (← ``(tryFinally $term $finallyTerm))
    let doElemsNew ← liftMacroM <| ToTerm.matchNestedTermResult term uvars a r bc
    doSeqToCode (doElemsNew ++ doElems)

partial def doSeqToCode : List Syntax → M CodeBlock
    | [] => do liftMacroM mkPureUnitAction
    | doElem::doElems => withIncRecDepth <| withRef doElem do
      checkSystem "`do`-expander"
      match (← liftMacroM <| expandMacro? doElem) with
      | some doElem => doSeqToCode (doElem::doElems)
      | none =>
      match (← liftMacroM <| expandDoIf? doElem) with
      | some doElem => doSeqToCode (doElem::doElems)
      | none =>
      match (← liftMacroM <| expandDoLetExpr? doElem doElems) with
      | some doElem => doSeqToCode [doElem]
      | none =>
        let (liftedDoElems, doElem) ← expandLiftMethod doElem
        if !liftedDoElems.isEmpty then
          doSeqToCode (liftedDoElems ++ [doElem] ++ doElems)
        else
          let ref := doElem
          let k := doElem.getKind
          if k == ``Parser.Term.doLet then
            let vars ← getDoLetVars doElem
            checkNotShadowingMutable vars
            mkVarDeclCore vars doElem <$> withNewMutableVars vars (isMutableLet doElem) (doSeqToCode doElems)
          else if k == ``Parser.Term.doHave then
            let vars ← getDoHaveVars doElem
            checkNotShadowingMutable vars
            mkVarDeclCore vars doElem <$> (doSeqToCode doElems)
          else if k == ``Parser.Term.doLetRec then
            let vars ← getDoLetRecVars doElem
            checkNotShadowingMutable vars
            mkVarDeclCore vars doElem <$> (doSeqToCode doElems)
          else if k == ``Parser.Term.doReassign then
            let vars ← getDoReassignVars doElem
            checkReassignable vars
            let k ← doSeqToCode doElems
            mkReassignCore vars doElem k
          else if k == ``Parser.Term.doLetArrow then
            doLetArrowToCode doElem doElems
          else if k == ``Parser.Term.doLetElse then
            doLetElseToCode doElem doElems
          else if k == ``Parser.Term.doReassignArrow then
            doReassignArrowToCode doElem doElems
          else if k == ``Parser.Term.doIf then
            doIfToCode doElem doElems
          else if k == ``Parser.Term.doUnless then
            doUnlessToCode doElem doElems
          else if k == ``Parser.Term.doFor then withFreshMacroScope do
            doForToCode doElem doElems
          else if k == ``Parser.Term.doMatch then
            doMatchToCode doElem doElems
          else if k == ``Parser.Term.doMatchExpr then
            doMatchExprToCode doElem doElems
          else if k == ``Parser.Term.doTry then
            doTryToCode doElem doElems
          else if k == ``Parser.Term.doBreak then
            ensureInsideFor
            ensureEOS doElems
            return mkBreak ref
          else if k == ``Parser.Term.doContinue then
            ensureInsideFor
            ensureEOS doElems
            return mkContinue ref
          else if k == ``Parser.Term.doReturn then
            doReturnToCode doElem doElems
          else if k == ``Parser.Term.doDbgTrace then
            return mkSeq doElem (← doSeqToCode doElems)
          else if k == ``Parser.Term.doAssert then
            return mkSeq doElem (← doSeqToCode doElems)
          else if k == ``Parser.Term.doDebugAssert then
            return mkSeq doElem (← doSeqToCode doElems)
          else if k == ``Parser.Term.doNested then
            let nestedDoSeq := doElem[1]
            doSeqToCode (getDoSeqElems nestedDoSeq ++ doElems)
          else if k == ``Parser.Term.doExpr then
            let term := doElem[0]
            if doElems.isEmpty then
              return mkTerminalAction term
            else
              return mkSeq term (← doSeqToCode doElems)
          else
            throwError "unexpected do-element of kind {doElem.getKind}:\n{doElem}"

def run (doStx : Syntax) (m : Syntax) (returnType : Syntax) : TermElabM CodeBlock :=
  (doSeqToCode <| getDoSeqElems <| getDoSeq doStx).run { ref := doStx, m, returnType }

def findNat (p : Nat -> Prop) [DecidablePred p] : Option Nat :=
  let rec aux i :=
    if p i then
      some i
    else
      aux (i + 1)
  partial_fixpoint
  aux 0

def find [Encodable α] (p : α -> Prop) [DecidablePred p] : Option α :=
  findNat (fun x => (Encodable.decode x).any (p ·)) |>.bind Encodable.decode

class Findable {α : Type u} (p : α -> Prop) where
  find : Unit -> Option α
  find_none : (find ()).isNone -> ∀ x, ¬ p x
  find_some_p : find () = some x -> p x

instance ExtractNonDet.ForIn_list {xs : List α} {init : β} {f : α → β → NonDetT m (ForInStep β)}
  (_ : ∀ a b, ExtractNonDet findable (f a b)) :
  ExtractNonDet findable (forIn xs init f) :=

@[simp, inline]
def NonDetT.extractGen {findable : {τ : Type u} -> (τ -> Prop) -> Type u} {α : Type u}
  (findOf : ∀ {τ : Type u} (p : τ -> Prop), findable p -> Unit -> Option τ)
  : (s : NonDetT m α) -> (ex : ExtractNonDet findable s := by admit /- proof elided -/
  ) -> m α
  | .pure x, _ => Pure.pure x
  | .vis x f, .vis _ _ _ => liftM x >>= (fun x => extractGen findOf (f x))
  | .pickCont _ p f, .pickSuchThat _ _ _ _ =>
    match findOf p ‹_› () with
    | none => CCPOBot.compBot
    | some x =>  extractGen findOf (f x)
  | .pickCont _ p f, .assume _ _ _ =>
    if p .unit then
      extractGen findOf (f .unit)
    else CCPOBot.compBot

@[inline]
def NonDetT.extract {α : Type u} (s : NonDetT m α) (ex : ExtractNonDet Findable s := by admit /- proof elided -/
) : m α :=
  NonDetT.extractGen Findable.find s

def findNat (p : Nat -> Prop) [DecidablePred p] : Option Nat :=
  let rec aux i :=
    if p i then
      some i
    else
      aux (i + 1)
  partial_fixpoint
  aux 0

def find [Encodable α] (p : α -> Prop) [DecidablePred p] : Option α :=
  findNat (fun x => (Encodable.decode x).any (p ·)) |>.bind Encodable.decode

class Findable {α : Type u} (p : α -> Prop) where
  find : Unit -> Option α
  find_none : (find ()).isNone -> ∀ x, ¬ p x
  find_some_p : find () = some x -> p x

def VelvetM.extract {α : Type} (x : VelvetM α)
  [∀ α, Lean.Order.CCPO (DivM α)] [Lean.Order.MonoBind DivM] [Inhabited α] : α :=
  x.run.run

inductive DivM (α : Type u) where
  | res (x : α)
  | div

def DivM.run {α : Type u} [Inhabited α] : DivM α -> α
  | DivM.res x => x
  | DivM.div => default

class CCPOBot (m : Type u -> Type v)  where
  compBot {α} : m α

def NonDetT.run {α : Type u} :  NonDetT m α -> m α
  | .pure x => Pure.pure x
  | .vis x f => liftM x >>= (fun x => (f x).run)
  | @NonDetT.pickCont _ _ _ p _ f =>
    match Findable.find (p := p) () with
    | none => CCPOBot.compBot
    | some x =>  (f x).run
  | .repeatCont init f cont =>
    forIn Lean.Loop.mk init (fun _ x => (f x).run) >>= (fun x => (cont x).run)

inductive NonDetT (m : Type u -> Type v) : (α : Type u) -> Type _ where
  | pure {α} (ret : α) : NonDetT m α
  | vis {α} {β} (x : m β) (f : β → NonDetT m α) : NonDetT m α
  | pickCont {α} (τ : Type u) (p : τ -> Prop) [Findable p] (f : τ → NonDetT m α) : NonDetT m α
  | repeatCont {α} {β} (init : β) (f : β -> NonDetT m (ForInStep β)) (cont : β -> NonDetT m α) : NonDetT m α

class MonadNonDet (m : Type u → Type v) where
  pick : (τ : Type u) → [Inhabited τ] → m τ
  
  pickSuchThat : (τ : Type u) → (p : τ → Prop) → [Findable p] → m τ
  assume : (as : Prop) → [Decidable as] → m PUnit.{u+1}
  
  rep {α : Type u} : α → (α → m (ForInStep α)) → m α
</all_available_defs>

<local_ctx>
import Auto

import Loom.MonadAlgebras.NonDetT.Extract

import Loom.MonadAlgebras.WP.Tactic

import Loom.MonadAlgebras.WP.DoNames'

import CaseStudies.Velvet.Std

import CaseStudies.TestingUtil

open PartialCorrectness DemonicChoice

section RunLengthEncoding

structure Encoding where
  cnt: Nat
  c: Char
  deriving Inhabited

def get_cnt_sum (l: List Encoding) :=
  match l with
  | List.nil => 0
  | List.cons x xs => x.cnt + get_cnt_sum xs

@[reducible]
def is_valid_run_sequence (encoded_str: Array Encoding) :=
    forall i, ( h: i < encoded_str.size ) -> (encoded_str[i]'h).cnt > 0

method decodeStr' (encoded_str: Array Encoding)
   return (res: Array Char)
   require is_valid_run_sequence encoded_str
   ensures (res.size = get_cnt_sum encoded_str.toList)
     do
       let mut decoded := Array.replicate 0 'x'
       let mut i := 0
       while i < encoded_str.size
          invariant 0 <= i ∧ i <= encoded_str.size
          invariant decoded.size = get_cnt_sum (encoded_str.extract 0 i).toList
          done_with i = encoded_str.size
          do
            let elem := encoded_str[i]!
            let elem_decoded := Array.replicate elem.cnt elem.c
            decoded :=  decoded ++ elem_decoded
            i := i + 1
       return decoded

prove_correct decodeStr' by
  loom_solve
  · simp[*] at *
    have : decoded.size = get_cnt_sum (List.take i encoded_str.toList) := by admit /- proof elided -/

@[grind]
def decodeStrLean (encoded_str: Array Encoding) : Array Char :=
  let mp := Array.map (fun e => Array.replicate e.cnt e.c) encoded_str
  mp.flatten
</local_ctx>

<target_theorem>
lemma array_extract_split_i_j_k (arr : Array α) (i j k: Nat) :
    i < j -> j < k -> k ≤ arr.size →
    arr.extract i k = (arr.extract i j) ++ (arr.extract j k) :=
</target_theorem>

