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>
Fin in Init.Prelude
Subspace in Mathlib.Algebra.Module.Submodule.Basic
Set in Mathlib.Data.Set.Defs
Set.Ico in Mathlib.Order.Interval.Set.Defs
Submodule in Mathlib.Algebra.Module.Submodule.Defs
Submodule.span in Mathlib.LinearAlgebra.Span.Defs
Polynomial.C in Mathlib.Algebra.Polynomial.Basic
Nat in Init.Prelude
Fintype in Mathlib.Data.Fintype.Defs
Fintype.card in Mathlib.Data.Fintype.Card
algebraMap in Mathlib.Algebra.Algebra.Defs
Fin.foldl in Init.Data.Fin.Fold
context in Examples.FrankingProtocol
Finset in Mathlib.Data.Finset.Defs
Finset.univ in Mathlib.Data.Fintype.Defs
Eq in Init.Prelude
Polynomial in Mathlib.Algebra.Polynomial.Basic
DecidableEq in Init.Prelude
</used_lib_defs>

<used_repo_defs>
noncomputable def W (i : Fin r) : L[X] :=
  ∏ u : U 𝔽q β i, (X - C u.val)

def U (i : Fin r) : Subspace 𝔽q L := Submodule.span 𝔽q (β '' (Set.Ico 0 i))

noncomputable def normalizedW (i : Fin r) : L[X] :=
  C (1 / (W 𝔽q β i).eval (β i)) * W 𝔽q β i

def getBit (k n : Nat) : Nat := (n >>> k) &&& 1

def g (n : ℕ) (c : ℕ) (x : ℕ) := (x * x + c) % n
</used_repo_defs>

<lib_lemmas>
Polynomial.comp_assoc in Mathlib.Algebra.Polynomial.Eval.Defs
implies_true in Init.SimpLemmas
Fin.coe_ofNat_eq_mod in Mathlib.Data.Fin.Basic
Fin.foldl_succ in Init.Data.Fin.Fold
Fin.val_succ in Init.Data.Fin.Lemmas
Nat.zero_mod in Init.Data.Nat.Div.Basic
Polynomial.X_comp in Mathlib.Algebra.Polynomial.Eval.Defs
Polynomial.comp_X in Mathlib.Algebra.Polynomial.Eval.Defs
add_zero in Mathlib.Algebra.Group.Defs
Finset.prod_congr in Mathlib.Algebra.BigOperators.Group.Finset.Basic
Polynomial.pow_comp in Mathlib.Algebra.Polynomial.Eval.Defs
Polynomial.prod_comp in Mathlib.Algebra.Polynomial.Eval.Defs
mul_comm in Mathlib.Algebra.Group.Defs
one_mul in Mathlib.Algebra.Group.Defs
pow_zero in Mathlib.Algebra.Group.Defs
</lib_lemmas>

<repo_lemmas>
lemma Xⱼ_zero_eq_one (ℓ : ℕ) (h_ℓ : ℓ ≤ r) :
  Xⱼ 𝔽q β ℓ h_ℓ ⟨0, by exact Nat.two_pow_pos ℓ⟩ = 1

lemma getBit_zero_of_two_mul {n : ℕ} : getBit 0 (2*n) = 0

lemma lt_two_pow_of_lt_two_pow_exp_le (x i j: ℕ)
    (h_x_lt_2_pow_i: x < 2^i) (h_i_le_j: i ≤ j): x < 2^j

lemma getBit_eq_succ_getBit_of_mul_two {n k : ℕ} : getBit (k+1) (2*n) = getBit k n

lemma getBit_of_shiftRight {n p : ℕ}:
  ∀ k, getBit k (n >>> p) = getBit (k+p) n

theorem mul_two_add_bit_lt_two_pow (a b c : ℕ) (i : Fin 2)
    (h_a : a < 2 ^ b) (h_b : b < c) :
    a * 2 + i.val < 2^c
</repo_lemmas>

<local_ctx>
import ArkLib.Data.FieldTheory.AdditiveNTT.NovelPolynomialBasis

import Mathlib.Tactic

import Mathlib.Data.Finsupp.Defs

import Mathlib.LinearAlgebra.LinearIndependent.Defs

open Polynomial AdditiveNTT Module

namespace AdditiveNTT

variable {r : ℕ} [NeZero r]

variable {L : Type u} [Field L] [Fintype L] [DecidableEq L]

variable (𝔽q : Type u) [Field 𝔽q] [Fintype 𝔽q] [DecidableEq 𝔽q]
  [h_Fq_char_prime : Fact (Nat.Prime (ringChar 𝔽q))] [hF₂ : Fact (Fintype.card 𝔽q = 2)]

variable [Algebra 𝔽q L]

variable (β : Fin r → L) [hβ_lin_indep : Fact (LinearIndependent 𝔽q β)]
  [h_β₀_eq_1 : Fact (β 0 = 1)]

variable {ℓ R_rate : ℕ} (h_ℓ_add_R_rate : ℓ + R_rate < r)-- ℓ ∈ {1, ..., r-1}

section IntermediateStructures

noncomputable def qMap (i : Fin r) : L[X] :=
  let constMultiplier := ((W 𝔽q β i).eval (β i))^(Fintype.card 𝔽q)
    / ((W 𝔽q β (i + 1)).eval (β (i + 1)))
  C constMultiplier * ∏ c: 𝔽q, (X - C (algebraMap 𝔽q L c))

noncomputable section DomainBijection

end DomainBijection

noncomputable def intermediateNormVpoly
    
    (i: Fin (ℓ+1)) (k : Fin (ℓ - i + 1)) : L[X] :=
  
  Fin.foldl (n:=k) (fun acc j =>
    (qMap 𝔽q β ⟨(i : ℕ) + (j : ℕ), by admit /- proof elided -/
    ⟩).comp acc) (X)

noncomputable def intermediateNovelBasisX (i : Fin (ℓ + 1)) (j : Fin (2 ^ (ℓ - i))) : L[X] :=
  (Finset.univ: Finset (Fin (ℓ - i)) ).prod (fun k =>
    (intermediateNormVpoly 𝔽q β h_ℓ_add_R_rate i (k:=⟨k, by admit /- proof elided -/
    ⟩)) ^ (Nat.getBit k j))
</local_ctx>

<target_theorem>
lemma even_index_intermediate_novel_basis_decomposition (i : Fin ℓ) (j : Fin (2 ^ (ℓ - i - 1))) :
  intermediateNovelBasisX 𝔽q β h_ℓ_add_R_rate ⟨i, by omega⟩ ⟨j * 2, by
    apply mul_two_add_bit_lt_two_pow j (ℓ-i-1) (ℓ-i) ⟨0, by omega⟩ (by omega) (by omega)
  ⟩  = (intermediateNovelBasisX 𝔽q β h_ℓ_add_R_rate ⟨i+1, by omega⟩ ⟨j, by
    apply lt_two_pow_of_lt_two_pow_exp_le j (ℓ-i-1) (ℓ-(i+1)) (by omega) (by omega)
  ⟩).comp (qMap 𝔽q β ⟨i, by omega⟩) :=
</target_theorem>

