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:
* <used_lib_defs>...</used_lib_defs>: Imported library definitions directly used by the theorem.
* <used_repo_defs>...</used_repo_defs>: Definitions from the local project (but outside the current file), directly or transitively used by the theorem.
* <lib_lemmas>...</lib_lemmas>: Imported library lemmas/theorems that may assist the proof.
* <repo_lemmas>...</repo_lemmas>: Lemmas/theorems defined in the local project outside this file.
* <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 used definitions and helpful lemmas (used_lib_defs, used_repo_defs, lib_lemmas, repo_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:
<used_lib_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)
</used_lib_defs>

<repo_lemmas>
theorem List.getElem_append_left{α : Type u_1} {l₁ l₂ : List α} {i : Nat} (hn : i < l₁.length) :\n(l₁ ++ l₂)[i] = l₁[i]
</repo_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:
<used_lib_defs>
DecidableEq in Init.Prelude
Lean.ToExpr in Lean.ToExpr
List in Init.Prelude
Repr in Init.Data.Repr
Nat in Init.Prelude
AList in Mathlib.Data.List.AList
Option in Init.Prelude
StateT in Init.Control.State
Unit in Init.Prelude
AList.insert in Mathlib.Data.List.AList
String in Init.Prelude
BEq in Init.Prelude
ite in Init.Prelude
Vector in Init.Data.Vector.Basic
Bool in Init.Prelude
Int in Init.Data.Int.Basic
bool in Init.Control.Basic
Computation in Mathlib.Data.Seq.Computation
Valuation.map in Mathlib.RingTheory.Valuation.Basic
Sigma in Init.Core
Sigma.mk in Init.Core
liftM in Init.Prelude
Iff in Init.Core
IsEmpty in Mathlib.Logic.IsEmpty
</used_lib_defs>

<used_repo_defs>
syntax "neg" : MLIR.Pretty.uniform_op

macro_rules
  | `(mlir_op| $res:mlir_op_operand = const ($x)
      $[: $outer_type]? ) => do
      let outer_type ← outer_type.getDM `(mlir_type| _)
      `(mlir_op| $res:mlir_op_operand = "const"()
          {val = $x:num : $outer_type} : ($outer_type) -> ($outer_type) )

macro_rules
  | `(mlir_op| $res:mlir_op_operand = li ($x)
     $[: $outer_type]?  ) => do
      let outer_type ← outer_type.getDM `(mlir_type| _ )
      `(mlir_op| $res:mlir_op_operand = "li"()
          {imm = $x:num : $outer_type } : ($outer_type) -> ($outer_type))

macro_rules
| `(mlir_op| $res:mlir_op_operand = $op1:MLIR.Pretty.RV.opWithImmediate $reg1 , $x : $t)  => do
    let some opName := MLIR.EDSL.Pretty.extractOpName op1.raw
      | Macro.throwUnsupported
    `(mlir_op| $res:mlir_op_operand = $opName ($reg1) {imm = $x:num : $t} : ($t) -> ($t) )

macro_rules
| `(mlir_op| $res:mlir_op_operand = $op1:MLIR.Pretty.RV.opWithShamt $reg1 , $x  : $t )  => do
    let some opName := MLIR.EDSL.Pretty.extractOpName op1.raw
      | Macro.throwUnsupported
    `(mlir_op| $res:mlir_op_operand = $opName ($reg1) {shamt = $x:num : $t}  : ($t) -> ($t) )

abbrev Mapping (Γ Δ : Ctxt Ty) : Type :=
  @AList (Σ t, Var Γ t) (fun x => Var Δ x.1)

structure Ctxt (Ty : Type) : Type where
  ofList :: toList : List Ty
  
  deriving Repr, Lean.ToExpr, DecidableEq

def Var (Γ : Ctxt Ty) (t : Ty) : Type :=
  { i : Nat // Γ[i]? = some t }

inductive Lets (Γ_in : Ctxt d.Ty) (eff : EffectKind) :
    (Γ_out : Ctxt d.Ty) → Type where
  | nil : Lets Γ_in eff Γ_in
  | var (body : Lets Γ_in eff Γ_out) (e : Expr d Γ_out eff t) : Lets Γ_in eff e.outContext

protected instance Expr.decidableEq [DecidableEq d.Op] [DecidableEq d.Ty] :
    {Γ : Ctxt d.Ty} → {ty : List d.Ty} → DecidableEq (Expr d Γ eff ty)
  | Γ, _, .mk op₁ rfl eff_le₁ arg₁ regArgs₁, .mk op₂ eq eff_le₂ arg₂ regArgs₂ =>
    if ho : op₁ = op₂ then by
      subst ho
      letI := HVector.decidableEq arg₁ arg₂
      letI := HVector.decidableEqReg regArgs₁ regArgs₂
      exact decidable_of_iff (arg₁ = arg₂ ∧ regArgs₁ = regArgs₂) (by admit /- proof elided -/
      )
    else isFalse (by admit /- proof elided -/
    )

def Var (Γ : Context Ty) (ty : Ty) : Type := { v : VarName // Γ.hasType v ty }

def Valuation.nil : Ctxt.Valuation (∅ : Ctxt Ty) := fun _ v => v.emptyElim

inductive EffectKind
| pure 
| impure 
deriving Repr, DecidableEq, Lean.ToExpr

@[elab_as_elim]
def appendCases
    {motive : (Γ ++ Δ).Var t → Sort u}
    (left : (v : Var Γ t) → motive (appendInl v))
    (right : (v : Var Δ t) → motive (appendInr v)) :
    (v : (Γ ++ Δ).Var t) → motive v
  | ⟨idx, h⟩ =>
    if hv : idx < Γ.length then
      left ⟨idx, by admit /- proof elided -/
      ⟩
    else
      let v' : Var _ _ := ⟨idx - Γ.length, by admit /- proof elided -/
      ⟩
      have eq : v'.appendInr = ⟨idx, h⟩ := by admit /- proof elided -/

@[match_pattern]
def last (Γ : Ctxt Ty) (t : Ty) : Ctxt.Var (Ctxt.cons t Γ) t :=
  ⟨0, by admit /- proof elided -/
  ⟩

def Valuation.cons {Γ : Ctxt Ty} {t : Ty} (x : toType t) (V : Γ.Valuation) :
    (Γ.cons t).Valuation :=

protected instance HVector.decidableEqReg [DecidableEq d.Op] [DecidableEq d.Ty] :
    ∀ {l : RegionSignature d.Ty}, DecidableEq (HVector (fun t => Com d t.1 .impure t.2) l)
  | _, .nil, .nil => isTrue rfl
  | _, .cons x₁ v₁, .cons x₂ v₂ =>
    letI := HVector.decidableEqReg v₁ v₂
    letI := Com.decidableEq x₁ x₂
    decidable_of_iff (x₁ = x₂ ∧ v₁ = v₂) (by admit /- proof elided -/
    )

def appendInl (v : Γ.Var t) : (Γ ++ Δ).Var t :=
  ⟨v.val, by admit /- proof elided -/
  ⟩

def sig          := Signature.sig ∘ s.signature

structure Signature (Ty : Type) where
  mkEffectful ::
  sig         : List Ty
  regSig      : RegionSignature Ty
  returnTypes : List Ty
  effectKind  : EffectKind := .pure

class DialectSignature (d : Dialect) where
  signature : d.Op → Signature d.Ty

def effectKind   := Signature.effectKind ∘ s.signature

def returnTypes  := Signature.returnTypes ∘ s.signature

def Expr.ty : Expr d Γ eff [t] → d.Ty := fun _ => t

def Expr.op {Γ : Ctxt d.Ty} {eff : EffectKind} {ty} (e : Expr d Γ eff ty) : d.Op :=
  Expr.casesOn e (fun op _ _ _ _ => op)

infixr:50 "::ₕ" => HVector.cons

infixr:67 "::ᵥ" => Valuation.cons
</used_repo_defs>

<lib_lemmas>
Bool.false_eq_true in Init.Data.Bool
IsEmpty.exists_iff in Mathlib.Logic.IsEmpty
Option.isSome_none in Init.Data.Option.Basic
iff_false in Init.SimpLemmas
Option.get_some in Init.Data.Option.Basic
Option.isSome_iff_exists in Init.Data.Option.Lemmas
forall_exists_index in Init.PropLemmas
</lib_lemmas>

<repo_lemmas>
theorem eq.ty_eq {v : Γ.Var t} {w : Γ.Var u} (h : v.eq w) : t = u

@[simp]
theorem Expr.op_mk {Γ : Ctxt d.Ty} {ty} {eff : EffectKind} (op : d.Op)
    (ty_eq : ty = DialectSignature.returnTypes op)
    (eff_le : DialectSignature.effectKind op ≤ eff)
    (args : HVector (Var Γ) (DialectSignature.sig op))
    (regArgs) :
    (Expr.mk op ty_eq eff_le args regArgs).op = op

@[simp]
theorem Expr.regArgs_mk {Γ : Ctxt d.Ty} {ty eff op}
    (ty_eq : ty = DialectSignature.returnTypes op)
    (eff_le : DialectSignature.effectKind op ≤ eff)
    (args : HVector (Var Γ) (DialectSignature.sig op)) (regArgs) :
    (Expr.mk op ty_eq eff_le args regArgs).regArgs = regArgs

@[simp] theorem appendCases_appendInl (v : Γ.Var t) :
    appendCases (motive := motive) left right v.appendInl = (left v)

@[simp]
theorem Expr.args_mk {Γ : Ctxt d.Ty} {ty eff op}
    (ty_eq : ty = DialectSignature.returnTypes op)
    (eff_le : DialectSignature.effectKind op ≤ eff)
    (args : HVector (Var Γ) (DialectSignature.sig op)) (regArgs) :
    (Expr.mk op ty_eq eff_le args regArgs).args = args
</repo_lemmas>

<local_ctx>
import LeanMLIR.Framework

import LeanMLIR.Transforms.Rewrite.Mapping

open Ctxt (Var VarSet Valuation Hom)

variable {d} [DialectSignature d] [DecidableEq d.Ty]

variable {Γ : Ctxt d.Ty} {ty : d.Ty}

abbrev MatchVarM (Δ Γ : Ctxt d.Ty) := (StateT (Mapping Δ Γ) Option)

abbrev MatchVar (Δ Γ : Ctxt d.Ty)  := MatchVarM Δ Γ Unit

def MatchVarM.unifyVars {Δ Γ : Ctxt d.Ty} (v : Δ.Var t) (w : Γ.Var t) : MatchVar Δ Γ :=
  fun ma =>
    match ma.lookup ⟨_, v⟩ with
    | some v =>
      if v = w then
        some ((), ma)
      else
        none
    | none =>
      some ((), AList.insert ⟨_, v⟩ w ma)

open MatchVarM

variable [DecidableEq d.Op]

def matchArg [DecidableEq d.Op]
    (lets : Lets d Γ_in eff Γ_out) (matchLets : Lets d Δ_in .pure Δ_out) :
    {l : List d.Ty} → HVector (Var Γ_out) l → HVector (Var Δ_out) l →
    MatchVar Δ_in Γ_out
  | _, .nil, .nil => return
  | t::l, .cons vₗ vsₗ, .cons vᵣ vsᵣ => do
      matchVar (t := t) lets vₗ matchLets vᵣ
      matchArg lets matchLets vsₗ vsᵣ
  termination_by l => (sizeOf matchLets, l.length + 1)

def matchVar {Γ_in Γ_out Δ_in Δ_out : Ctxt d.Ty} {t : d.Ty} [DecidableEq d.Op]
    (lets : Lets d Γ_in eff Γ_out) (v : Var Γ_out t) :
    (matchLets : Lets d Δ_in .pure Δ_out) →
    (w : Var Δ_out t) →
    MatchVar Δ_in Γ_out
   
  | @Lets.var _ _ _ _ Δ_out ts matchLets matchExpr, w => by admit /- proof elided -/
      | right w =>
        exact matchVar lets v matchLets w
      | left w => exact do
        let ⟨ts', w', ie⟩ ← lets.getPureExpr v
        if hs : ∃ h : ie.op = matchExpr.op, ie.regArgs = (h ▸ matchExpr.regArgs) then
          have hts : Ctxt.ofList ts' = ts := by admit /- proof elided -/

end

section MatchVar

variable [DecidableEq d.Op] {Γ_in Γ_out Δ_in Δ_out t te}
          {lets : Lets d Γ_in eff Γ_out} {v : Var Γ_out t}
          {matchLets : Lets d Δ_in .pure Δ_out}
          {matchExpr : Expr d Δ_out .pure te}
</local_ctx>

<target_theorem>
theorem matchVar_appendInl {w : Var ⟨te⟩ t} :
    matchVar lets v (.var matchLets matchExpr) w.appendInl ma = some ma' →
    ∃ args,
      lets.getPureExpr v
        = some ⟨_, w, matchExpr.op, matchExpr.ty_eq, matchExpr.eff_le, args, matchExpr.regArgs⟩
      ∧ matchArg lets matchLets args matchExpr.args ma = some ma' :=
</target_theorem>

