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>
Array in Init.Prelude
Array.emptyWithCapacity in Init.Prelude
Nat in Init.Prelude
BEq in Init.Prelude
DecidableEq in Init.Prelude
FinEnum in Mathlib.Data.FinEnum
Hashable in Init.Prelude
Repr in Init.Data.Repr
Std.HashMap in Std.Data.HashMap.Basic
Std.HashSet in Std.Data.HashSet.Basic
BitVec in Init.Prelude
Prod in Init.Prelude
Prod.mk in Init.Prelude
Bool in Init.Prelude
Classical.propDecidable in Init.Classical
Decidable in Init.Prelude
NFA in Mathlib.Computability.NFA
Set in Mathlib.Data.Set.Defs
SetRel in Mathlib.Data.Rel
List in Init.Prelude
Subtype in Init.Prelude
LawfulBEq in Init.Core
Function.Injective2 in Mathlib.Logic.Function.Basic
List.Nodup in Init.Data.List.Basic
FinEnum.toList in Mathlib.Data.FinEnum
</used_lib_defs>

<used_repo_defs>
structure CNFA (n : Nat) where
  m : RawCNFA (BitVec n)
  wf : m.WF

instance RawCNFA.statesFinset (m : RawCNFA A) : Fintype m.states := (Finset.range m.stateMax).fintypeCoeSort

abbrev State := Nat

def product (final? : Prop → Prop → Prop) (M N : NFA' n) : NFA' n where
  σ := _
  M := M.M.product final? N.M

structure NFA' (n : Nat) where
  σ : Type
  M : NFA (BitVec n) σ

def nfa : NFA A S where
  start := { sa | sa ∈ inits }
  accept := { sa | final sa }
  step sa a := { sa' | (a, sa') ∈ f sa }

def Std.HashSet.toSet [BEq α] [Hashable α] (m : HashSet α) : Set α := { x | x ∈ m }

axiom hashMap_missing : ∀ {P : Prop}, P
</used_repo_defs>

<lib_lemmas>
Array.mem_push in Init.Data.Array.Lemmas
Array.mem_toList_iff in Init.Data.Array.Lemmas
Array.toList_push in Init.Data.Array.Bootstrap
Function.Injective2.eq_iff in Mathlib.Logic.Function.Basic
List.Nodup.append in Mathlib.Data.List.Nodup
List.append_assoc in Init.Data.List.Basic
List.disjoint_singleton in Batteries.Data.List.Lemmas
List.mem_append in Init.Data.List.Lemmas
List.mem_singleton in Init.Data.List.Lemmas
Set.mem_empty_iff_false in Mathlib.Data.Set.Basic
Set.mem_insert_iff in Mathlib.Data.Set.Insert
Set.union_singleton in Mathlib.Data.Set.Insert
and_false in Init.SimpLemmas
iff_and_self in Init.SimpLemmas
ne_eq in Init.SimpLemmas
or_false in Init.SimpLemmas
true_and in Init.SimpLemmas
ne_or_eq in Mathlib.Logic.Basic
Array.emptyWithCapacity_eq in Init.Data.Array.Basic
and_self in Init.SimpLemmas
implies_true in Init.SimpLemmas
Array.mem_def in Init.Data.Array.Basic
Array.not_mem_empty in Init.Data.Array.Lemmas
FinEnum.nodup_toList in Mathlib.Data.FinEnum
List.Nodup.notMem in Mathlib.Data.List.Nodup
List.dedup_eq_self in Mathlib.Data.List.Dedup
List.foldl_nil in Init.Data.List.Basic
List.mem_cons in Init.Data.List.Lemmas
List.nodup_cons in Init.Data.List.Pairwise
List.not_mem_nil in Init.Data.List.Lemmas
false_and in Init.SimpLemmas
false_or in Init.SimpLemmas
Set.mem_setOf_eq in Mathlib.Data.Set.Operations
</lib_lemmas>

<repo_lemmas>
theorem Std.HashSet.fold_induction [BEq α] [LawfulBEq α] [Hashable α]
  {f : β → α → β} {m : HashSet α} {motive : β → Set α → Prop} :
    motive b ∅ →
    (∀ b x s, x ∉ s → motive b s → motive (f b x) (s ∪ {x})) →
    motive (m.fold f b) m.toSet

theorem Std.HashSet.toSet_toList[BEq α] [LawfulBEq α] [Hashable α] (m : HashSet α) : m.toSet = { x | x ∈ m.toList }

@[simp]
lemma Std.HashSet.mem_toSet [BEq α] [Hashable α] (m : HashSet α) : x ∈ m.toSet ↔ x ∈ m

@[simp]
theorem Std.HashSet.mem_attachWith_mem [BEq α] [Hashable α] [LawfulBEq α] (m : HashSet α) {P H} (x : α) h :
    ⟨x, h⟩ ∈ m.attachWith P H ↔ x ∈ m
</repo_lemmas>

<local_ctx>
import Mathlib.Data.Fintype.Prod

import Blase.AutoStructs.Worklist

import Mathlib.Tactic.ApplyFun

open SetRel

section sink

variable {A : Type} [BEq A] [Hashable A] [DecidableEq A] [FinEnum A]

end sink

section generic_prod

variable {α} [BEq α] [Hashable α] [LawfulBEq α]

variable {β} [BEq β] [Hashable β] [LawfulBEq β]

variable {S₁ : Finset α} {S₂ : Finset β}

variable {γ} (f : S₁ → S₂ → γ) (hinj : Function.Injective2 f)

variable {m₁ : Std.HashSet α} (hm₁ : ∀ s₁ ∈ m₁, s₁ ∈ S₁)

variable {m₂ : Std.HashSet β} (hm₂ : ∀ s₂ ∈ m₂, s₂ ∈ S₂)

@[inline]
def product.prodArray' (a : Array γ) :=
  m₁.attachWith _ hm₁ |>.fold (init := a) fun is s1 =>
    m₂.attachWith _ hm₂ |>.fold (init := is) fun is s2 =>
      is.push (f s1 s2)

@[inline]
def product.prodArray := prodArray' f hm₁ hm₂ (Array.emptyWithCapacity <| m₁.size * m₂.size)

end generic_prod

section product

variable {A : Type} [BEq A] [LawfulBEq A] [Hashable A] [DecidableEq A] [FinEnum A]

def product.inits (m₁ m₂ : CNFA n) :=
  product.prodArray Prod.mk @m₁.wf.initials_lt @m₂.wf.initials_lt

def product (final? : Bool → Bool → Bool) (m₁ m₂ : CNFA n) : CNFA n :=
  worklistRun (m₁.m.states × m₂.m.states) final (product.inits m₁ m₂)
    (by admit /- proof elided -/
    ) f
where final (ss : m₁.m.states × m₂.m.states) := final? (ss.1 ∈ m₁.m.finals) (ss.2 ∈ m₂.m.finals)
      f (ss : m₁.m.states × m₂.m.states) :=
        let (s1, s2) := ss
        (FinEnum.toList (α := BitVec n)).foldl (init := Array.empty) fun as a =>
          product.prodArray' (λ s₁ s₂ ↦ (a, (s₁, s₂)))
            (fun s' => m₁.wf.trans_tgt_lt (s := s1) (a := a)) (fun s' => m₂.wf.trans_tgt_lt (s := s2) (a := a)) as

noncomputable def to_prop (f : Bool → Bool → Bool) (p1 p2 : Prop) : Prop :=
  f (@Decidable.decide p1 (Classical.propDecidable _)) (@Decidable.decide p2 (Classical.propDecidable _))
</local_ctx>

<target_theorem>
lemma product.sim {m1 m2 : CNFA n}:
    m1.Sim M1 → m2.Sim M2 →
    (nfa (product.inits m1 m2) (final final? m1 m2) (f m1 m2)).Bisim (M1.M.product (to_prop final?) M2.M) :=
</target_theorem>

