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>
Field in Mathlib.Algebra.Field.Defs
Finset in Mathlib.Data.Finset.Defs
Finset.range in Mathlib.Data.Finset.Range
Polynomial.C in Mathlib.Algebra.Polynomial.Basic
Polynomial.X in Mathlib.Algebra.Polynomial.Basic
AdjoinRoot.instField in Mathlib.RingTheory.AdjoinRoot
Fintype in Mathlib.Data.Fintype.Defs
Fintype.card in Mathlib.Data.Fintype.Card
Irreducible in Mathlib.Algebra.Group.Irreducible.Defs
List in Init.Prelude
List.Vector in Mathlib.Data.Vector.Defs
NeZero in Init.Data.NeZero
Polynomial in Mathlib.Algebra.Polynomial.Basic
AdjoinRoot in Mathlib.RingTheory.AdjoinRoot
AdjoinRoot.of in Mathlib.RingTheory.AdjoinRoot
AdjoinRoot.root in Mathlib.RingTheory.AdjoinRoot
Eq in Init.Prelude
Eq.mp in Init.Core
CharP in Mathlib.Algebra.CharP.Defs
DivisionRing in Mathlib.Algebra.Field.Defs
DivisionSemiring in Mathlib.Algebra.Field.Defs
Finset.Icc in Mathlib.Order.Interval.Finset.Defs
GroupWithZero in Mathlib.Algebra.GroupWithZero.Defs
Nat in Init.Prelude
Ne in Init.Core
Equiv in Mathlib.Logic.Equiv.Defs
Equiv.ofBijective in Mathlib.Logic.Equiv.Defs
Fact in Mathlib.Logic.Basic
Fintype.ofEquiv in Mathlib.Data.Fintype.OfMap
Function.Bijective in Mathlib.Logic.Function.Defs
Function.Injective in Init.Data.Function
Function.Surjective in Init.Data.Function
Polynomial.Monic in Mathlib.Algebra.Polynomial.Degree.Definitions
Polynomial.eval₂ in Mathlib.Algebra.Polynomial.Eval.Defs
Prod in Init.Prelude
Semiring in Mathlib.Algebra.Ring.Defs
Fintype.ofFinite in Mathlib.Data.Fintype.EquivFin
List.Vector.cons in Mathlib.Data.Vector.Defs
List.Vector.nil in Mathlib.Data.Vector.Defs
Algebra in Mathlib.Algebra.Algebra.Defs
algebraMap in Mathlib.Algebra.Algebra.Defs
instAlgebra in Mathlib.LinearAlgebra.TensorAlgebra.Basic
RingHom in Mathlib.Algebra.Ring.Hom.Defs
RingHom.id in Mathlib.Algebra.Ring.Hom.Defs
Algebra.algebraMap in Mathlib.Algebra.Algebra.Defs
reduceDIte in Lean.Meta.Tactic.Simp.BuiltinSimprocs.Core
CommRing in Mathlib.Algebra.Ring.Defs
PowerBasis in Mathlib.RingTheory.PowerBasis
Fin in Init.Prelude
cast in Init.Prelude
</used_lib_defs>

<used_repo_defs>
notation : 10 "GF(" term : 10 ")" => GaloisField term 1

structure TraceMapProperty (F : Type*) [Field F] (u : F) (k : ℕ) : Prop where
  element_trace : ∑ i ∈ Finset.range (2 ^ k), u ^ (2 ^ i) = 1
  inverse_trace : ∑ i ∈ Finset.range (2 ^ k), (u⁻¹) ^ (2 ^ i) = 1

noncomputable def definingPoly {F : Type*} [instField : Field F] (s : F)
  := X ^ 2 + C s * X + 1

def AlgebraTowerEquiv.symm (e : AlgebraTowerEquiv A B) : AlgebraTowerEquiv B A where
  toRingEquiv := fun i => (e.toRingEquiv i).symm
  commutesLeft' := fun i j h r => by admit /- proof elided -/

structure AlgebraTowerEquiv (A : ι → Type*) [∀ i, CommSemiring (A i)] [a : AlgebraTower A]
  (B : ι → Type*) [∀ i, CommSemiring (B i)] [b : AlgebraTower B]
  where
    toRingEquiv : ∀ i, (A i ≃+* B i)
    commutesLeft' : ∀ (i j : ι) (h : i ≤ j) (r : A i),
      (b.algebraMap (i:=i) (j:=j) (h:=h)) ((toRingEquiv i) r) =
      (toRingEquiv j) (a.algebraMap (i:=i) (j:=j) (h:=h) r)

structure SpecialElementRelation {F_prev : Type*} [Field F_prev] (t1 : F_prev)
  {F_cur : Type*} [Field F_cur] (u : F_cur) [Algebra F_prev F_cur] : Prop where
    sum_inv_eq : u + u⁻¹ = algebraMap F_prev F_cur t1
    h_u_square : u^2 = u * (algebraMap F_prev F_cur t1) + 1

instance irreducible_quadratic_defining_poly_of_traceMap_eq_1
  {F : Type*} [Field F] [Fintype F] [CharP F 2] (s : F) [NeZero s] (k : ℕ)
  (trace_map_prop : TraceMapProperty F s k)
  (fintypeCard : Fintype.card F = 2 ^ (2 ^ k))
  : Irreducible (definingPoly s) :=

instance charP_eq_2_of_add_self_eq_zero {F : Type*} [Field F]
    (sumZeroIffEq : ∀ (x y : F), x + y = 0 ↔ x = y) : CharP F 2 :=
  have h_two : (2 : (F)) = 0 := by admit /- proof elided -/

def coeff.{u} {F : Type u} [Semiring F] (f : F[X][Y]) (i j : ℕ) : F := (f.coeff j).coeff i

instance GF_2_fintype : Fintype (GF(2)) := Fintype.ofFinite (GF(2))

@[simp]
def AlgebraTower.toAlgebra {i j : ι} (h : i ≤ j) : Algebra (A i) (A j) :=
  (AlgebraTower.algebraMap (i:=i) (j:=j) (h:=h)).toAlgebra
</used_repo_defs>

<lib_lemmas>
Ne.dite_eq_left_iff in Mathlib.Logic.Basic
Nat.add_one_sub_one in Init.Data.Nat.Basic
Nat.left_eq_add in Init.Data.Nat.Lemmas
RingHom.comp_id in Mathlib.Algebra.Ring.Hom.Defs
cast_eq in Init.Core
eq_mp_eq_cast in Init.PropLemmas
one_ne_zero in Mathlib.Algebra.NeZero
Polynomial.natDegree_eq_of_degree_eq_some in Mathlib.Algebra.Polynomial.Degree.Definitions
Algebra.smul_def in Mathlib.Algebra.Algebra.Defs
eqRec_eq_cast in Batteries.Logic
Algebra.smul_def' in Mathlib.Algebra.Algebra.Defs
Nat.sub_one_add_one in Init.Data.Nat.Basic
cast_cast in Init.PropLemmas
map_zero in Mathlib.Algebra.Group.Hom.Defs
zero_add in Mathlib.Algebra.Group.Defs
Function.comp_apply in Init.Core
Polynomial.ext in Mathlib.Algebra.Polynomial.Basic
RingHom.coe_comp in Mathlib.Algebra.Ring.Hom.Defs
</lib_lemmas>

<repo_lemmas>
lemma degree_definingPoly {F : Type*} [Field F] [Fintype F] (s : F) [NeZero s] :
  (definingPoly s).degree = 2

lemma degree_s_smul_X_add_1 {F : Type*} [Field F] [Fintype F] (s : F) [NeZero s] :
  (C s * (X : (F)[X]) + 1).degree = 1

lemma degree_s_smul_X {F : Type*} [Field F] [Fintype F] (s : F) [NeZero s] :
  (C s * (X : (F)[X])).degree = 1

theorem unique_linear_form_of_elements_in_adjoined_commring
    {R : Type*} [CommRing R] (f : R[X]) (hf_deg : f.natDegree = 2)
    (hf_monic : Monic f) (c1 : AdjoinRoot f) :
    ∃! p : R × R, c1 = (AdjoinRoot.of f) p.1 * root f + (AdjoinRoot.of f) p.2

theorem unique_linear_sum_repr (R : Type*) [CommRing R] (S : Type*) [CommRing S] [Algebra R S]
    (pb : PowerBasis R S) (h_dim : pb.dim = 2) (s : S) :
    ∃! p : R × R, s = p.fst • pb.gen + algebraMap R S p.snd

theorem unique_repr {R : Type*} [CommRing R] {S : Type*} [CommRing S] [Algebra R S]
    (pb : PowerBasis R S) (repr1 repr2 : Fin pb.dim →₀ R)
    (h : ∑ i : Fin pb.dim, repr1 i • pb.basis i = ∑ i : Fin pb.dim, repr2 i • pb.basis i) :
    repr1 = repr2
</repo_lemmas>

<local_ctx>
import ArkLib.Data.FieldTheory.BinaryField.Tower.Prelude

import ArkLib.Data.RingTheory.AlgebraTower

import Mathlib.Tactic.DepRewrite

namespace BinaryTower

noncomputable section

open Polynomial AdjoinRoot Module

section BTFieldDefs

structure BinaryTowerResult (F : Type _) (k : ℕ) where
  vec : (List.Vector F (k + 1))
  instField : (Field F)
  instFintype : Fintype F
  specialElement : F
  specialElementNeZero : NeZero specialElement
  firstElementOfVecIsSpecialElement [Inhabited F] : vec.1.headI = specialElement
  instIrreduciblePoly : (Irreducible (p := (definingPoly specialElement)))
  sumZeroIffEq : ∀ (x y : F), x + y = 0 ↔ x = y
  fieldFintypeCard : Fintype.card F = 2^(2^k)
  traceMapEvalAtRootsIs1 : TraceMapProperty F specialElement k

structure BinaryTowerInductiveStepResult (k : ℕ) (prevBTField : Type _)
  (prevBTResult : BinaryTowerResult prevBTField k) [instPrevBTFieldIsField : Field prevBTField]
  (prevPoly : Polynomial prevBTField) (F : Type _) where
  binaryTowerResult : BinaryTowerResult F (k+1)
  eq_adjoin : F = AdjoinRoot prevPoly
  u_is_root : Eq.mp (eq_adjoin) binaryTowerResult.specialElement = AdjoinRoot.root prevPoly
  eval_defining_poly_at_root : Eq.mp (eq_adjoin) binaryTowerResult.specialElement^2 +
    Eq.mp (eq_adjoin) binaryTowerResult.specialElement * (of prevPoly) prevBTResult.specialElement
    + 1 = 0

def binary_tower_inductive_step
  (k : Nat)
  (prevBTField : Type _) [Field prevBTField]
  (prevBTResult : BinaryTowerResult prevBTField k)
: Σ' (F : Type _), BinaryTowerInductiveStepResult (k:=k) (prevBTField:=prevBTField)
  (prevBTResult:=prevBTResult) (prevPoly:=definingPoly (F:=prevBTField)
    (instField:=prevBTResult.instField) (s:=prevBTResult.specialElement)) (F:=F)
  (instPrevBTFieldIsField:=prevBTResult.instField) :=

def BinaryTowerAux (k : ℕ) : (Σ' (F : Type 0), BinaryTowerResult F k) :=
  match k with
  | 0 => 
    let curBTField := GF(2)
    let newList : List.Vector (GF(2)) 1 := List.Vector.cons (1 : GF(2)) List.Vector.nil
    let specialElement : GF(2) := newList.1.headI
    let firstElementOfVecIsSpecialElement : newList.1.headI = specialElement := rfl
    let specialElementIs1 : specialElement = 1 := by admit /- proof elided -/

@[simp]
def BTField (k : ℕ) := (BinaryTowerAux k).1

@[simp]
instance Inhabited (k : ℕ) : Inhabited (BTField k) where
  default := (0 : BTField k)

@[simp]
def sumZeroIffEq (k : ℕ) : ∀ (x y : BTField k),
  x + y = 0 ↔ x = y := (BinaryTowerAux k).2.sumZeroIffEq

@[simp]
def Z (k : ℕ) : BTField k := (BinaryTowerAux k).snd.specialElement 

@[simp]
def poly (k : ℕ) : Polynomial (BTField k) := definingPoly (s:=(Z k))

instance polyMonic (n : ℕ) : Monic (poly n) := definingPoly_is_monic (Z n)

end BTFieldDefs

section BinaryAlgebraTower

def canonicalEmbedding (k : ℕ) : BTField k →+* BTField (k+1) :=
  AdjoinRoot.of (poly k)

def towerAlgebraMap (l r : ℕ) (h_le : l ≤ r) : BTField l →+* BTField r :=

def binaryAlgebraTower {l r : ℕ} (h_le : l ≤ r) : Algebra (BTField l) (BTField r) :=

instance (priority := 1000) algebra_adjacent_tower (l : ℕ) :
  Algebra (BTField l) (BTField (l+1)) :=

end BinaryAlgebraTower

noncomputable section MultilinearBasis

def join_via_add_smul {k : ℕ} (h_pos : k > 0) (hi_btf lo_btf : BTField (k - 1)) :
    BTField k :=

def split (k : ℕ) (h_k : k > 0) (x : BTField k) : BTField (k-1) × BTField (k-1) :=
</local_ctx>

<target_theorem>
lemma algebraMap_eq_zero_x {i j : ℕ} (h_le : i < j) (x : BTField i) :
    letI instAlgebra :=
</target_theorem>

