-- Lean 4 proof step from Mathlib.RingTheory.WittVector.Frobenius (goal)
Goal:
p : ℕ
hp : Fact (Nat.Prime p)
n : ℕ
IH :
  ∀ m < n,
    HPow.hPow (α := MvPolynomial ℕ ℚ) (X m) p +
        HMul.hMul (α := MvPolynomial ℕ ℚ) (β := MvPolynomial ℕ ℚ)
          ((C : (a : ℚ) → MvPolynomial ℕ ℚ) (↑p : ℚ) : MvPolynomial ℕ ℚ)
          ((MvPolynomial.map (Int.castRingHom ℚ) : (a : MvPolynomial ℕ ℤ) → MvPolynomial ℕ ℚ) (frobeniusPolyAux p m) :
            MvPolynomial ℕ ℚ) =
      (bind₁ (wittPolynomial p ℚ ∘ fun (n : ℕ) ↦ n + 1) : (a : MvPolynomial ℕ ℚ) → MvPolynomial ℕ ℚ) (xInTermsOfW p ℚ m)
h1 :
  HPow.hPow (α := ℚ) (↑p : ℚ) n *
      HPow.hPow (α := ℚ) (@Invertible.invOf _ Distrib.toMul AddMonoidWithOne.toOne (↑p : ℚ) _) n =
    1
⊢ HMul.hMul (α := MvPolynomial ℕ ℚ) (β := MvPolynomial ℕ ℚ)
        (∑ x ∈ range n,
            HMul.hMul (α := MvPolynomial ℕ ℚ)
              ((C : (a : ℚ) → MvPolynomial ℕ ℚ) (HPow.hPow (α := ℚ) (↑p : ℚ) x) : MvPolynomial ℕ ℚ)
              (HPow.hPow (α := MvPolynomial ℕ ℚ)
                ((bind₁ (wittPolynomial p ℚ ∘ fun (n : ℕ) ↦ n + 1) : (a : MvPolynomial ℕ ℚ) → MvPolynomial ℕ ℚ)
                    (xInTermsOfW p ℚ x) :
                  MvPolynomial ℕ ℚ)
                (p ^ (n - x))) :
          MvPolynomial ℕ ℚ)
        ((C : (a : ℚ) → MvPolynomial ℕ ℚ)
            (HPow.hPow (α := ℚ) (@Invertible.invOf _ Distrib.toMul AddMonoidWithOne.toOne (↑p : ℚ) _) n) :
          MvPolynomial ℕ ℚ) -
      HMul.hMul (α := MvPolynomial ℕ ℚ) (β := MvPolynomial ℕ ℚ)
        (∑ x ∈ range n,
            HMul.hMul (α := MvPolynomial ℕ ℚ)
              ((C : (a : ℚ) → MvPolynomial ℕ ℚ) (HPow.hPow (α := ℚ) (↑p : ℚ) x) : MvPolynomial ℕ ℚ)
              (HPow.hPow (α := MvPolynomial ℕ ℚ) (X x) (p ^ (n + 1 - x))) :
          MvPolynomial ℕ ℚ)
        ((C : (a : ℚ) → MvPolynomial ℕ ℚ)
            (HPow.hPow (α := ℚ) (@Invertible.invOf _ Distrib.toMul AddMonoidWithOne.toOne (↑p : ℚ) _) n) :
          MvPolynomial ℕ ℚ) =
    HMul.hMul (α := MvPolynomial ℕ ℚ) ((C : (a : ℚ) → MvPolynomial ℕ ℚ) (↑p : ℚ))
      ((MvPolynomial.map (Int.castRingHom ℚ) : (a : MvPolynomial ℕ ℤ) → MvPolynomial ℕ ℚ)
        (∑ i ∈ range n,
          ∑ j ∈ range (p ^ (n - i)),
            HPow.hPow (α := MvPolynomial ℕ ℤ) (X i) p ^ (p ^ (n - i) - (j + 1)) * frobeniusPolyAux p i ^ (j + 1) *
              (C : (a : ℤ) → MvPolynomial ℕ ℤ)
                (↑((p ^ (n - i)).choose (j + 1) / p ^ (n - i - v p (j + 1)) * p ^ (j - v p (j + 1))) : ℤ)))
have simp : Eq (α := MvPolynomial ℕ ℚ)
  (∑ x ∈ range n,
    (HMul.hMul (β := MvPolynomial ℕ ℚ)
        (HMul.hMul (α := MvPolynomial ℕ ℚ)
          ((C : (a : ℚ) → MvPolynomial ℕ ℚ) (HPow.hPow (α := ℚ) (↑p : ℚ) x) : MvPolynomial ℕ ℚ)
          (HPow.hPow (α := MvPolynomial ℕ ℚ)
            ((bind₁ (wittPolynomial p ℚ ∘ fun (n : ℕ) ↦ n + 1) : (a : MvPolynomial ℕ ℚ) → MvPolynomial ℕ ℚ)
                (xInTermsOfW p ℚ x) :
              MvPolynomial ℕ ℚ)
            (p ^ (n - x))))
        ((C : (a : ℚ) → MvPolynomial ℕ ℚ)
            (HPow.hPow (α := ℚ) (@Invertible.invOf _ Distrib.toMul AddMonoidWithOne.toOne (↑p : ℚ) _) n) :
          MvPolynomial ℕ ℚ) -
      HMul.hMul (β := MvPolynomial ℕ ℚ)
        (HMul.hMul (α := MvPolynomial ℕ ℚ)
          ((C : (a : ℚ) → MvPolynomial ℕ ℚ) (HPow.hPow (α := ℚ) (↑p : ℚ) x) : MvPolynomial ℕ ℚ)
          (HPow.hPow (α := MvPolynomial ℕ ℚ) (X x) (p ^ (n + 1 - x))))
        ((C : (a : ℚ) → MvPolynomial ℕ ℚ)
            (HPow.hPow (α := ℚ) (@Invertible.invOf _ Distrib.toMul AddMonoidWithOne.toOne (↑p : ℚ) _) n) :
          MvPolynomial ℕ ℚ)))
  (∑ x ∈ range n,
    ∑ i ∈ range (p ^ (n - x)),
      HMul.hMul (α := MvPolynomial ℕ ℚ) (β := MvPolynomial ℕ ℚ)
        ((C : (a : ℚ) → MvPolynomial ℕ ℚ) (↑p : ℚ) : MvPolynomial ℕ ℚ)
        ((MvPolynomial.map (Int.castRingHom ℚ) : (a : MvPolynomial ℕ ℤ) → MvPolynomial ℕ ℚ)
            (HPow.hPow (α := MvPolynomial ℕ ℤ) (X x) p ^ (p ^ (n - x) - (i + 1)) * frobeniusPolyAux p x ^ (i + 1) *
              (C : (a : ℤ) → MvPolynomial ℕ ℤ)
                (↑((p ^ (n - x)).choose (i + 1) / p ^ (n - x - v p (i + 1)) * p ^ (i - v p (i + 1))) : ℤ)) :
          MvPolynomial ℕ ℚ))
Tactic: simp only [map_sum, mul_sum, sum_mul, ← sum_sub_distrib]