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
Vector in Init.Data.Vector.Basic
BitVec in Init.Prelude
BitVec.ofFin in Init.Prelude
Nat in Init.Prelude
Vector.ofFn in Init.Data.Vector.Basic
List in Init.Prelude
List.ofFn in Init.Data.List.OfFn
Nat.succ in Init.Prelude
NeZero in Init.Data.NeZero
Fin.isValue in Lean.Meta.Tactic.Simp.BuiltinSimprocs.Fin
reduceCtorEq in Lean.Meta.Tactic.Simp.BuiltinSimprocs.Core
Bool in Init.Prelude
List.foldr in Init.Data.List.Basic
List.foldl in Init.Prelude
</used_lib_defs>

<used_repo_defs>
@[elab_as_elim] def Fin.succRecOnSameFinType {motive : Fin r → Sort _}
    (zero : motive (0 : Fin r))
    (succ : ∀ i : Fin r, i + 1 < r → motive i → motive (i + 1)) : ∀ (i : Fin r), motive i
  | ⟨0, _⟩ => by admit /- proof elided -/
  | ⟨Nat.succ i_val, h⟩ => by admit /- proof elided -/

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

<lib_lemmas>
List.length_ofFn in Init.Data.List.OfFn
List.getElem_ofFn in Init.Data.List.OfFn
List.get_eq_getElem in Init.Data.List.Lemmas
Fin.is_lt in Init.Data.Fin.Lemmas
Fin.eta in Init.Data.Fin.Lemmas
List.ext_getElem in Init.Data.List.Lemmas
List.getElem_dropLast in Init.Data.List.Lemmas
List.length_dropLast in Init.Data.List.Lemmas
add_tsub_cancel_right in Mathlib.Algebra.Order.Sub.Defs
Fin.coe_ofNat_eq_mod in Mathlib.Data.Fin.Basic
Fin.val_succ in Init.Data.Fin.Lemmas
List.ofFn_succ in Init.Data.List.OfFn
Nat.zero_mod in Init.Data.Nat.Div.Basic
add_zero in Mathlib.Algebra.Group.Defs
ne_eq in Init.SimpLemmas
not_false_eq_true in Init.SimpLemmas
BitVec.getElem_ofFin in Init.Data.BitVec.Lemmas
BitVec.getLsb_eq_getElem in Init.Data.BitVec.Basic
Bool.false_eq_true in Init.Data.Bool
Fin.getElem_fin in Init.GetElem
Nat.sub_lt_of_lt in Init.Data.Nat.Basic
Vector.getElem_ofFn in Init.Data.Vector.OfFn
add_sub_assoc in Mathlib.Algebra.Group.Defs
sub_self in Mathlib.Algebra.Group.Defs
Fin.cast_succ_eq in Init.Data.Fin.Lemmas
Fin.cast_zero in Init.Data.Fin.Lemmas
Fin.coe_cast in Init.Data.Fin.Lemmas
Fin.eq_mk_iff_val_eq in Init.Data.Fin.Lemmas
Fin.mk_zero' in Mathlib.Data.Fin.Basic
Fin.val_eq_zero in Init.Data.Fin.Lemmas
List.foldl_cons in Init.Data.List.Basic
List.foldl_nil in Init.Data.List.Basic
List.foldr_cons in Init.Data.List.Basic
List.foldr_nil in Init.Data.List.Basic
List.ofFn_zero in Init.Data.List.OfFn
Nat.lt_of_lt_of_eq in Init.Data.Nat.Basic
Nat.mod_succ in Init.Data.Nat.Lemmas
tsub_self in Mathlib.Algebra.Order.Sub.Basic
zero_add in Mathlib.Algebra.Group.Defs
</lib_lemmas>

<repo_lemmas>
lemma testBit_true_eq_getBit_eq_1 (k n : Nat) : n.testBit k = ((Nat.getBit k n) = 1)

lemma testBit_false_eq_getBit_eq_0 (k n : Nat) :
  (n.testBit k = false) = ((Nat.getBit k n) = 0)

lemma getBit_of_sub_two_pow_of_bit_1 {n i j: ℕ} (h_getBit_eq_1: getBit i n = 1) :
  getBit j (n - 2^i) = (if j = i then 0 else getBit j n)

lemma and_two_pow_eq_two_pow_of_getBit_1 {n i : ℕ} (h_getBit: getBit i n = 1) :
    n &&& (2 ^ i) = 2 ^ i

lemma getBit_of_xor {n m k: ℕ} : getBit k (n ^^^ m) = getBit k n ^^^ getBit k m

lemma getBit_two_pow {i k : ℕ} : (getBit k (2^i) = if i == k then 1 else 0)

lemma xor_eq_sub_iff_submask {n m : ℕ} (h: m ≤ n) : n ^^^ m = n - m ↔ n &&& m = m

lemma sum_eq_xor_plus_twice_and (n : Nat) : ∀ m : ℕ, n + m = (n ^^^ m) + 2 * (n &&& m)

lemma and_by_split_lowBits {n m n1 m1 bn bm : ℕ} (h_bn : bn < 2) (h_bm : bm < 2)
  (h_n : n = n1 * 2 + bn) (h_m : m = m1 * 2 + bm):
  n &&& m = (n1 &&& m1) * 2 + (bn &&& bm)

lemma div_2_form {nD2 b : ℕ} (h_b : b < 2):
  (nD2 * 2 + b) / 2 = nD2

lemma or_by_split_lowBits {n m n1 m1 bn bm : ℕ} (h_bn : bn < 2) (h_bm : bm < 2)
  (h_n : n = n1 * 2 + bn) (h_m : m = m1 * 2 + bm):
  n ||

lemma xor_by_split_lowBits {n m n1 m1 bn bm : ℕ} (h_bn : bn < 2) (h_bm : bm < 2)
  (h_n : n = n1 * 2 + bn) (h_m : m = m1 * 2 + bm):
  n ^^^ m = (n1 ^^^ m1) * 2 + (bn ^^^ bm)

lemma Fin.val_add_one' (a : Fin r) (h_a_add_1 : a + 1 < r) : (a + 1).val = a.val + 1

theorem foldr_split_inner {α : Type u} {β : Type v} (f : α → β → β) (init : β)
    (l : List α) (h : l ≠ []): foldr (f:=f) (init:=init) (l)
    = foldr (f:=f) (init:=f (l.getLast (by omega)) (init)) (l.dropLast)

theorem append_getLast_dropLast {α : Type u} (l : List α) (h : l ≠ []) :
  l.dropLast ++ [l.getLast h] = l

theorem foldl_split_outer {α : Type u} {β : Type v} (f : α → β → α) (init : α)
    (l : List β) (h : l ≠ []): foldl (f:=f) (init:=init) (l)
    = f (foldl (f:=f) (init:=init) (l.dropLast)) (l.getLast (by omega))
</repo_lemmas>

<local_ctx>
import ArkLib.Data.Nat.Bitwise

import Mathlib.RingTheory.MvPolynomial.Basic

import ToMathlib.General

import ArkLib.Data.Fin.BigOperators

import ArkLib.Data.List.Lemmas

import ArkLib.Data.Vector.Basic

@[reducible]
def MlPoly (R : Type*) (n : ℕ) := Vector R (2 ^ n) 

variable {R : Type*} {n : ℕ}

namespace MlPoly

section MlPolyInstances

end MlPolyInstances

section MlPolyMonomialBasisAndEvaluations

variable [CommRing R]

variable {S : Type*} [CommRing S]

variable {S : Type*} [CommRing S]

end MlPolyMonomialBasisAndEvaluations

end MlPoly

namespace MlPolyEval

section MlPolyEvalInstances

end MlPolyEvalInstances

section MlPolyLagrangeBasisAndEvaluations

variable [CommRing R]

variable {S : Type*} [CommRing S]

variable {S : Type*} [CommRing S]

end MlPolyLagrangeBasisAndEvaluations

end MlPolyEval

namespace MlPoly

variable {R : Type*} [AddCommGroup R]

@[inline] def monoToLagrangeLevel {n : ℕ} (j : Fin n) : Vector R (2 ^ n) → Vector R (2 ^ n) :=
  fun v =>
    let stride : ℕ := 2 ^ j.val    
    Vector.ofFn (fun i : Fin (2 ^ n) =>
      if (BitVec.ofFin i).getLsb j then
        v[i] + v[i - stride]'(Nat.sub_lt_of_lt i.isLt)
      else
        v[i])

@[inline] def lagrangeToMonoLevel {n : ℕ} (j : Fin n) : Vector R (2 ^ n) → Vector R (2 ^ n) :=
  fun v =>
    let stride : ℕ := 2 ^ j.val  
    Vector.ofFn (fun i : Fin (2 ^ n) =>
      if (BitVec.ofFin i).getLsb j then
        v[i] - v[i - stride]'(Nat.sub_lt_of_lt i.isLt)
      else
        v[i])

def forwardRange (n : ℕ) (r : Fin (n)) (l : Fin (r.val + 1)) : List (Fin n) :=
  let len := r.val - l.val + 1
  List.ofFn (fun (k : Fin len) =>
    let val := l.val + k.val
    have h_bound : val < n := by admit /- proof elided -/
  )

def monoToLagrange_segment (n : ℕ) (r : Fin n) (l : Fin (r.val + 1)) :
    Vector R (2 ^ n) → Vector R (2 ^ n) :=
  let range := forwardRange n r l
  (range.foldl (fun acc h => monoToLagrangeLevel h acc))

def lagrangeToMono_segment (n : ℕ) (r : Fin n) (l : Fin (r.val + 1)) :
    Vector R (2 ^ n) → Vector R (2 ^ n) :=
  let range := forwardRange n r l
  (range.foldr (fun h acc => lagrangeToMonoLevel h acc))
</local_ctx>

<target_theorem>
theorem mobius_apply_zeta_apply_eq_id (n : ℕ) [NeZero n] (r : Fin n) (l : Fin (r.val + 1))
    (v : Vector R (2 ^ n)) : lagrangeToMono_segment n r l (monoToLagrange_segment n r l v) = v :=
</target_theorem>

