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>
BitVec in Init.Prelude
Nat in Init.Prelude
List in Init.Prelude
List.Vector in Mathlib.Data.Vector.Defs
Set in Mathlib.Data.Set.Defs
NFA in Mathlib.Computability.NFA
DecidableEq in Init.Prelude
Fintype in Mathlib.Data.Fintype.Defs
Repr in Init.Data.Repr
NFA.accept in Mathlib.Computability.NFA
NFA.start in Mathlib.Computability.NFA
NFA.step in Mathlib.Computability.NFA
Unit in Init.Prelude
cmp in Mathlib.Data.Ordering.Basic
BitVec.ofNat in Init.Prelude
NFA.stepSet in Mathlib.Computability.NFA
BitVec.cons in Init.Data.BitVec.Basic
Language in Mathlib.Computability.Language
List.Vector.ofFn in Mathlib.Data.Vector.Defs
List.Vector.replicate in Mathlib.Data.Vector.Defs
Bool in Init.Prelude
Fin in Init.Prelude
BitVec.iunfoldr in Init.Data.BitVec.Folds
List.finRange in Init.Data.List.FinRange
List.reverseRecOn in Mathlib.Data.List.Induction
FinEnum in Mathlib.Data.FinEnum
BitVec.ofFin in Init.Prelude
BitVec.ule in Init.Data.BitVec.Basic
BitVec.ult in Init.Data.BitVec.Basic
BitVec.sle in Init.Data.BitVec.Basic
BitVec.slt in Init.Data.BitVec.Basic
</used_lib_defs>

<used_repo_defs>
structure BitVecs (n : Nat) where
  w : Nat
  bvs : List.Vector (BitVec w) n

inductive RelationOrdering
| lt | le | gt | ge
deriving Repr, Fintype

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

inductive Relation
| eq
| signed (ord : RelationOrdering)
| unsigned (ord : RelationOrdering)
deriving Repr

def evalRelation (rel : Relation) {w} (bv1 bv2 : BitVec w) : Prop :=
  match rel with
  | .eq => bv1 = bv2
  | .signed .lt => bv1.slt bv2
  | .signed .le => bv1.sle  bv2
  | .signed .gt => bv2.slt bv1
  | .signed .ge => bv2.sle bv1
  | .unsigned .lt => bv1.ult bv2
  | .unsigned .le => bv1.ule bv2
  | .unsigned .gt => bv2.ult bv1
  | .unsigned .ge => bv2.ule bv1

def bv2 : BitVec 4 := BitVec.ofNat 4 1 

def bv1 : BitVec 4 := BitVec.ofNat 4 5 

def accepts (M : NFA' n) : Set (BitVecs n) := dec '' M.accepts'

def accepts' (M : NFA' n) : Set (BitVecs' n) := M.M.accepts

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

def BitVec.ofFn {w : Nat} (f : Fin w → Bool) : BitVec w :=
  BitVec.iunfoldr (fun i _ => ((), f i)) () |>.2

def BitStream : Type := Nat → Bool

def enc (bvs : BitVecs n) : BitVecs' n :=
  (List.finRange bvs.w).map (fun i =>
    BitVec.ofFn (fun (k : Fin n) => (bvs.bvs.get k)[i]))

instance instFinEnumBV : FinEnum (BitVec w) where
  card := 2^w
  equiv := {
    toFun := fun x => x.toFin
    invFun := fun x => BitVec.ofFin x
    left_inv := by admit /- proof elided -/
</used_repo_defs>

<lib_lemmas>
NFA.eval_append_singleton in Mathlib.Computability.NFA
NFA.eval_nil in Mathlib.Computability.NFA
BitVec.toNat_eq in Init.Data.BitVec.Lemmas
le_iff_lt_or_eq in Mathlib.Order.Defs.PartialOrder
Nat.le_antisymm in Init.Prelude
BitVec.toInt_inj in Init.Data.BitVec.Lemmas
</lib_lemmas>

<repo_lemmas>
@[ext]
theorem ext {x y : BitStream} (h : ∀ i, x i = y i) : x = y
</repo_lemmas>

<local_ctx>
import Batteries.Data.Fin.Basic

import Batteries.Data.Fin.Lemmas

import Blase.SingleWidth.Defs

import Blase.AutoStructs.Constructions

import Blase.AutoStructs.Defs

import Blase.AutoStructs.FiniteStateMachine

import Mathlib.Tactic.Ring

import Mathlib.Data.Nat.Size -- TODO: remove and get rid of shiftLeft_eq_mul_pow use

import Mathlib.Data.BitVec

import Mathlib.Tactic.FinCases

open Fin.NatCast

open Mathlib

def NFA.sa (_ : NFA α σ) := σ → Language α

structure NFA.correct (M : NFA α σ) (ζ : M.sa) (L : Language α) where
  cond1 : ∀ w, (w ∈ L ↔ ∃ q ∈ M.accept, w ∈ ζ q)
  cond2 : ∀ w q, q ∈ M.eval w ↔ w ∈ ζ q

abbrev BVRel := ∀ ⦃w⦄, BitVec w → BitVec w → Prop

abbrev BVNRel n := ∀ ⦃w⦄, List.Vector (BitVec w) n → Prop

def NFA'.sa (M : NFA' n) := M.σ → BVNRel n

def NFA'.sa2 (M : NFA' 2) := M.σ → BVRel

def langRel (R : BVNRel n) : Set (BitVecs n) :=
  { bvs | R bvs.bvs }

def langRel2 (R : BVRel) : Set (BitVecs 2) :=
  { bvs | R (bvs.bvs.get 0) (bvs.bvs.get 1) }

structure NFA'.correct (M : NFA' n) (ζ : M.sa) (L : BVNRel n) where
  cond1 : ∀ ⦃w⦄ (bvn : List.Vector (BitVec w) n), (L bvn ↔ ∃ q ∈ M.M.accept, ζ q bvn)
  cond2 q : q ∈ M.M.start ↔ ζ q (List.Vector.replicate n .nil)
  cond3 q a {w} (bvn : List.Vector (BitVec w) n) : q ∈ M.M.stepSet { q | ζ q bvn } a ↔
              ζ q (List.Vector.ofFn fun k => BitVec.cons (a.getLsbD k) (bvn.get k))

structure NFA'.correct2 (M : NFA' 2) (ζ : M.sa2) (L : BVRel) where
  cond1 : ∀ (bv1 bv2 : BitVec w), (L bv1 bv2 ↔ ∃ q ∈ M.M.accept, ζ q bv1 bv2)
  cond2 q : q ∈ M.M.start ↔ ζ q .nil .nil
  cond3 q a w (bv1 bv2 : BitVec w) : q ∈ M.M.stepSet { q | ζ q bv1 bv2 } a ↔
              ζ q (BitVec.cons (a.getLsbD 0) bv1) (BitVec.cons (a.getLsbD 1) bv2)

section fsm

variable {arity : Type} [FinEnum arity]

open BitStream in

end fsm

section nfas_relations

def NFA.autEq : NFA (BitVec 2) Unit :=
  { start := ⊤, accept := ⊤, step _ a := { _s' | if a = 0 ∨ a = 3 then true else false }}

def NFA'.autEq : NFA' 2 :=
  ⟨Unit, NFA.autEq⟩

def NFA'.eqRel : BVRel := fun _ x y => x = y

inductive NFA.unsignedCmpState : Type where
| eq | gt | lt
deriving Fintype, DecidableEq

def NFA.unsignedCmpStep (q : NFA.unsignedCmpState) (a : BitVec 2) : List NFA.unsignedCmpState :=
  match q, a with
  | .eq, 0 => [ .eq ] | .eq, 3 => [ .eq ] | .eq, 1 => [ .gt ] | .eq, 2 => [ .lt ]
  | .gt, 0 => [ .gt ] | .gt, 1 => [ .gt ] | .gt, 3 => [ .gt ] | .gt, 2 => [ .lt ]
  | .lt, 0 => [ .lt ] | .lt, 1 => [ .gt ] | .lt, 2 => [ .lt ] | .lt, 3 => [ .lt ]

def NFA.autUnsignedCmp (cmp: RelationOrdering) : NFA (BitVec 2) unsignedCmpState where
  step s a := { s' | s' ∈ unsignedCmpStep s a }
  start := {s | s = .eq }
  accept := { s | s ∈ match cmp with | .lt => [unsignedCmpState.lt] | .le => [.lt, .eq] | .gt => [.gt] | .ge => [.gt, .eq] }

def NFA'.autUnsignedCmp (cmp: RelationOrdering) : NFA' 2 :=
  ⟨_, NFA.autUnsignedCmp cmp⟩

def RelationOrdering.urel (cmp : RelationOrdering) : BVRel :=
  match cmp with
  | .lt => fun _ bv1 bv2 => bv1.ult bv2
  | .le => fun _ bv1 bv2 => bv1.ule bv2
  | .gt => fun _ bv1 bv2 => bv2.ult bv1
  | .ge => fun _ bv1 bv2 => bv2.ule bv1

def NFA'.autUnsignedCmpSA (q : NFA.unsignedCmpState) : BVRel :=
  match q with
  | .eq => fun _ bv1 bv2 => bv1 = bv2
  | .lt => fun _ bv1 bv2 => bv1.ult bv2
  | .gt => fun _ bv1 bv2 => bv2.ult bv1

inductive NFA.signedCmpState : Type where
| eq | gt | lt | ltfin | gtfin
deriving DecidableEq, Fintype

def NFA.signedCmpStep (q : NFA.signedCmpState) (a : BitVec 2) : List NFA.signedCmpState :=
  match q, a with
  | .eq, 0 => [ .eq ] | .eq, 3 => [.eq] | .eq, 1 => [.gt, .ltfin] | .eq, 2 => [ .lt, .gtfin ]
  | .gt, 0 => [ .gt, .gtfin ] | .gt, 1 => [ .gt, .ltfin ] | .gt, 3 => [ .gt, .gtfin ] | .gt, 2 => [ .lt, .gtfin ]
  | .lt, 0 => [ .lt, .ltfin ] | .lt, 1 => [ .gt, .ltfin ] | .lt, 2 => [ .lt, .gtfin ] | .lt, 3 => [ .lt, .ltfin ]
  | .gtfin, _ => ∅
  | .ltfin, _ => ∅

def NFA.autSignedCmp (cmp: RelationOrdering) : NFA (BitVec 2) signedCmpState where
  step s a := { s' | s' ∈ signedCmpStep s a }
  start := { s | s = signedCmpState.eq }
  accept := { s | s ∈ match cmp with | .lt => [NFA.signedCmpState.ltfin] | .le => [.ltfin, .eq] | .gt => [.gtfin] | .ge => [.gtfin, .eq] }

def NFA'.autSignedCmp (cmp: RelationOrdering) : NFA' 2 :=
  ⟨_, NFA.autSignedCmp cmp⟩

def RelationOrdering.srel (cmp : RelationOrdering) : BVRel :=
  match cmp with
  | .lt => fun _ bv1 bv2 => bv1.slt bv2
  | .le => fun _ bv1 bv2 => bv1.sle bv2
  | .gt => fun _ bv1 bv2 => bv2.slt bv1
  | .ge => fun _ bv1 bv2 => bv2.sle bv1

def NFA'.autSignedCmpSA (q : NFA.signedCmpState) : BVRel :=
  match q with
  | .eq => fun _ bv1 bv2 => bv1 = bv2
  | .lt => fun _ bv1 bv2 => bv1.ult bv2
  | .gt => fun _ bv1 bv2 => bv2.ult bv1
  | .ltfin => fun _ bv1 bv2 => bv1.slt bv2
  | .gtfin => fun _ bv1 bv2 => bv2.slt bv1

end nfas_relations

def Relation.absAutOfRelation (rel : Relation) : NFA' 2 :=
  match rel with
  | .eq => NFA'.autEq
  | .unsigned cmp => NFA'.autUnsignedCmp cmp
  | .signed cmp => NFA'.autSignedCmp cmp
</local_ctx>

<target_theorem>
@[simp]
lemma autOfRelation_accepts (r : Relation) :
    r.absAutOfRelation.accepts = r.language :=
</target_theorem>

