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
Matrix in Mathlib.LinearAlgebra.Matrix.Defs
Semiring in Mathlib.Algebra.Ring.Defs
Matrix.of in Mathlib.LinearAlgebra.Matrix.Defs
Polynomial.X in Mathlib.Algebra.Polynomial.Basic
toFun in ToMathlib.Control.Monad.Hom
Submodule in Mathlib.Algebra.Module.Submodule.Defs
Polynomial in Mathlib.Algebra.Polynomial.Basic
Polynomial.degreeLT in Mathlib.RingTheory.Polynomial.Basic
Fin.val in Init.Prelude
Finset in Mathlib.Data.Finset.Defs
Finset.map in Mathlib.Data.Finset.Image
Finset.max in Mathlib.Data.Finset.Max
Fintype in Mathlib.Data.Fintype.Defs
CommRing in Mathlib.Algebra.Ring.Defs
LinearMap in Mathlib.Algebra.Module.LinearMap.Defs
LinearMap.range in Mathlib.Algebra.Module.Submodule.Range
DecidableEq in Init.Prelude
Field in Mathlib.Algebra.Field.Defs
NeZero in Init.Data.NeZero
Matrix.add in Mathlib.LinearAlgebra.Matrix.Defs
Finset.range in Mathlib.Data.Finset.Range
CommSemiring in Mathlib.Algebra.Ring.Defs
Polynomial.coeff in Mathlib.Algebra.Polynomial.Basic
</used_lib_defs>

<used_repo_defs>
def polynomialOfCoeffs (coeffs : Fin deg → F) : F[X] :=
  ⟨
    Finset.map ⟨Fin.val, Fin.val_injective⟩ {i | coeffs i ≠ 0},
    fun i ↦ if h : i < deg then coeffs ⟨i, h⟩ else 0,
    fun a ↦ by admit /- proof elided -/
  ⟩

def liftF' (f : ℕ → α) : Fin n → α :=
  fun m ↦ f m.1

noncomputable def fromColGenMat [CommRing F] (G : Matrix ι κ F) : LinearCode ι F :=
  LinearMap.range G.mulVecLin

abbrev LinearCode.{u, v} (ι : Type u) [Fintype ι] (F : Type v) [Semiring F] : Type (max u v) :=
  Submodule F (ι → F)

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

<lib_lemmas>
Polynomial.mem_degreeLT in Mathlib.RingTheory.Polynomial.Basic
Polynomial.natDegree_lt_iff_degree_lt in Mathlib.Algebra.Polynomial.Degree.Definitions
Matrix.mulVecLin_apply in Mathlib.LinearAlgebra.Matrix.ToLin
Matrix.mulVec_eq_sum in Mathlib.Data.Matrix.Mul
Finset.mem_range in Mathlib.Data.Finset.Range
Finset.sum_congr in Mathlib.Algebra.BigOperators.Group.Finset.Basic
Finset.sum_fin_eq_sum_range in Mathlib.Data.Fintype.BigOperators
Polynomial.eval_eq_sum_range' in Mathlib.Algebra.Polynomial.Eval.Degree
Polynomial.ext in Mathlib.Algebra.Polynomial.Basic
LinearMap.mem_range in Mathlib.Algebra.Module.Submodule.Range
Submodule.mem_map in Mathlib.Algebra.Module.Submodule.Map
</lib_lemmas>

<repo_lemmas>
@[simp]
lemma liftF'_p_coeff {p : F[X]} {k : ℕ} {i : Fin k} : liftF' p.coeff i = p.coeff i

@[simp]
lemma coeff_polynomialOfCoeffs_eq_coeffs :
  Fin.liftF' (polynomialOfCoeffs coeffs).coeff = coeffs
</repo_lemmas>

<local_ctx>
import ArkLib.Data.MvPolynomial.LinearMvExtension

import ArkLib.Data.Polynomial.Interface

import Mathlib.LinearAlgebra.Lagrange

import Mathlib.RingTheory.Henselian

namespace ReedSolomon

open Polynomial NNReal

variable {F : Type*} {ι : Type*} (domain : ι ↪ F)

def evalOnPoints [Semiring F] : F[X] →ₗ[F] (ι → F) where
  toFun := fun p => fun x => p.eval (domain x)
  map_add' := fun x y => by admit /- proof elided -/

def code (deg : ℕ) [Semiring F]: Submodule F (ι → F) :=
  (Polynomial.degreeLT F deg).map (evalOnPoints domain)

variable [Semiring F]

end ReedSolomon

open Polynomial Matrix Code LinearCode

variable {F ι ι' : Type*}
         {C : Set (ι → F)}

noncomputable section

namespace Vandermonde

def nonsquare [Semiring F] (ι' : ℕ) (α : ι → F) : Matrix ι (Fin ι') F :=
  Matrix.of fun i j => (α i) ^ j.1

section

variable [CommRing F] {m n : ℕ} {α : Fin m → F}

section

variable [IsDomain F]

end

end

end Vandermonde

namespace ReedSolomonCode

section

open Finset Function

open scoped BigOperators

variable {ι : Type*} [Fintype ι] [Nonempty ι]
         {F : Type*} [Field F] [Fintype F]

open Classical in

end

section

variable {deg m n : ℕ} {α : Fin m → F}

section

variable [Semiring F] {p : F[X]}

end

open LinearCode
</local_ctx>

<target_theorem>
lemma genMatIsVandermonde [Fintype ι] [Field F] [DecidableEq F] [inst : NeZero m] {α : ι ↪ F} :
  fromColGenMat (Vandermonde.nonsquare (ι' := m) α) = ReedSolomon.code α m :=
</target_theorem>

