-- Mathlib/RingTheory/WittVector/FrobeniusFractionField.lean
theorem succNthVal_spec' (n : ℕ) (a₁ a₂ : 𝕎 k) (bs : Fin (n + 1) → k) (ha₁ : a₁.coeff 0 ≠ 0)
    (ha₂ : a₂.coeff 0 ≠ 0) :
    succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) +
          a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) +
        nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) =
      succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) +
          a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) +
        nthRemainder p n bs (truncateFun (n + 1) a₂)

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
n : ℕ
a₁ a₂ : 𝕎 k
bs : Fin (n + 1) → k
ha₁ : a₁.coeff 0 ≠ 0
ha₂ : a₂.coeff 0 ≠ 0
⊢ succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) + a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) +
      nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) =
    succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) + a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) +
      nthRemainder p n bs (truncateFun (n + 1) a₂)

Tactic:
rw [← sub_eq_zero]

Result:
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
n : ℕ
a₁ a₂ : 𝕎 k
bs : Fin (n + 1) → k
ha₁ : a₁.coeff 0 ≠ 0
ha₂ : a₂.coeff 0 ≠ 0
⊢ succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) + a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) +
        nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
      (succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) + a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) +
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    0

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
n : ℕ
a₁ a₂ : 𝕎 k
bs : Fin (n + 1) → k
ha₁ : a₁.coeff 0 ≠ 0
ha₂ : a₂.coeff 0 ≠ 0
this : (succNthDefiningPoly p n a₁ a₂ bs).IsRoot (succNthVal p n a₁ a₂ bs ha₁ ha₂)
⊢ succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) + a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) +
        nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
      (succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) + a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) +
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    0

Tactic:
simp only [Polynomial.map_add, Polynomial.eval_X, Polynomial.map_pow, Polynomial.eval_C,
  Polynomial.eval_pow, succNthDefiningPoly, Polynomial.eval_mul, Polynomial.eval_add,
  Polynomial.eval_sub, Polynomial.map_mul, Polynomial.map_sub, Polynomial.IsRoot.def]
  at this

Result:
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
n : ℕ
a₁ a₂ : 𝕎 k
bs : Fin (n + 1) → k
ha₁ : a₁.coeff 0 ≠ 0
ha₂ : a₂.coeff 0 ≠ 0
this :
  succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) -
        succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) +
      (a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) + nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
          a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) -
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    0
⊢ succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) + a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) +
        nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
      (succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) + a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) +
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    0

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
n : ℕ
a₁ a₂ : 𝕎 k
bs : Fin (n + 1) → k
ha₁ : a₁.coeff 0 ≠ 0
ha₂ : a₂.coeff 0 ≠ 0
⊢ succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) + a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) +
        nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
      (succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) + a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) +
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    0

Tactic:
have := succNthVal_spec p n a₁ a₂ bs ha₁ ha₂

Result:
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
n : ℕ
a₁ a₂ : 𝕎 k
bs : Fin (n + 1) → k
ha₁ : a₁.coeff 0 ≠ 0
ha₂ : a₂.coeff 0 ≠ 0
this : (succNthDefiningPoly p n a₁ a₂ bs).IsRoot (succNthVal p n a₁ a₂ bs ha₁ ha₂)
⊢ succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) + a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) +
        nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
      (succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) + a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) +
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    0

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
n : ℕ
a₁ a₂ : 𝕎 k
bs : Fin (n + 1) → k
ha₁ : a₁.coeff 0 ≠ 0
ha₂ : a₂.coeff 0 ≠ 0
this :
  succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) -
        succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) +
      (a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) + nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
          a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) -
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    0
⊢ succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) + a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) +
        nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
      (succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) + a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) +
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    0

Tactic:
convert this using 1

Result:
case h.e'_2
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
n : ℕ
a₁ a₂ : 𝕎 k
bs : Fin (n + 1) → k
ha₁ : a₁.coeff 0 ≠ 0
ha₂ : a₂.coeff 0 ≠ 0
this :
  succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) -
        succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) +
      (a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) + nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
          a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) -
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    0
⊢ succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) + a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) +
        nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
      (succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) + a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) +
        nthRemainder p n bs (truncateFun (n + 1) a₂)) =
    succNthVal p n a₁ a₂ bs ha₁ ha₂ ^ p * a₁.coeff 0 ^ p ^ (n + 1) -
        succNthVal p n a₁ a₂ bs ha₁ ha₂ * a₂.coeff 0 ^ p ^ (n + 1) +
      (a₁.coeff (n + 1) * (bs 0 ^ p) ^ p ^ (n + 1) + nthRemainder p n (fun v => bs v ^ p) (truncateFun (n + 1) a₁) -
          a₂.coeff (n + 1) * bs 0 ^ p ^ (n + 1) -
        nthRemainder p n bs (truncateFun (n + 1) a₂))