-- Lean 4 proof step from Mathlib.NumberTheory.Padics.Hensel (goal)
Goal:
p : ℕ
inst : Fact (Nat.Prime p)
R : Type u_1
inst_1 : CommSemiring R
inst_2 : Algebra R ℤ_[p]
F : Polynomial R
a : ℤ_[p]
hnorm :
  norm (E := ℤ_[p]) (((Polynomial.aeval a : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p]) F : ℤ_[p]) <
    HPow.hPow (β := ℕ)
      (norm (E := ℤ_[p])
        (((Polynomial.aeval a : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p])
            ((Polynomial.derivative : (a : Polynomial R) → Polynomial R) F) :
          ℤ_[p]))
      2
z z' z1 : ℤ_[p]
n : ℕ
hz : ih_gen.{u_1} (R := R) (F := F) (a := a) n z
q : ℤ_[p]
heq : ((Polynomial.aeval z' : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p]) F = q * HPow.hPow (β := ℕ) z1 2
h1 :
  ‖HDiv.hDiv (α := ℚ_[p]) (↑(((Polynomial.aeval z : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p]) F) : ℚ_[p])
        (↑(((Polynomial.aeval z : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p])
              ((Polynomial.derivative : (a : Polynomial R) → Polynomial R) F)) :
          ℚ_[p])‖ ≤
    1
hzeq :
  z1 =
    Subtype.mk (p := fun (x : ℚ_[p]) ↦ ‖x‖ ≤ 1)
      (HDiv.hDiv (α := ℚ_[p])
        (↑(((Polynomial.aeval z : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p]) F) : ℚ_[p])
        (↑(((Polynomial.aeval z : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p])
              ((Polynomial.derivative : (a : Polynomial R) → Polynomial R) F)) :
          ℚ_[p]))
      h1
by :
  norm (E := ℤ_[p]) (((Polynomial.aeval z' : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p]) F : ℤ_[p]) =
    ‖q‖ * HPow.hPow (β := ℕ) ‖z1‖ 2
by₁ : ‖q‖ * HPow.hPow (β := ℕ) ‖z1‖ 2 ≤ 1 * HPow.hPow (β := ℕ) ‖z1‖ 2
by₂ :
  1 * HPow.hPow (β := ℕ) ‖z1‖ 2 =
    HPow.hPow (β := ℕ)
        (norm (E := ℤ_[p]) (((Polynomial.aeval z : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p]) F : ℤ_[p]))
        2 /
      HPow.hPow (β := ℕ)
        (norm (E := ℤ_[p])
          (((Polynomial.aeval a : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p])
              ((Polynomial.derivative : (a : Polynomial R) → Polynomial R) F) :
            ℤ_[p]))
        2
⊢ norm (E := ℤ_[p]) (((Polynomial.aeval z' : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p]) F : ℤ_[p]) ≤
    HPow.hPow (β := ℕ)
        (norm (E := ℤ_[p])
          (((Polynomial.aeval a : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p])
              ((Polynomial.derivative : (a : Polynomial R) → Polynomial R) F) :
            ℤ_[p]))
        2 *
      T_gen p F a ^ HPow.hPow (α := ℕ) 2 (n + 1)
have by₃ : HPow.hPow (β := ℕ)
      (norm (E := ℤ_[p]) (((Polynomial.aeval z : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p]) F : ℤ_[p])) 2 /
    HPow.hPow (β := ℕ)
      (norm (E := ℤ_[p])
        (((Polynomial.aeval a : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p])
            ((Polynomial.derivative : (a : Polynomial R) → Polynomial R) F) :
          ℤ_[p]))
      2 ≤
  HPow.hPow (β := ℕ)
      (HPow.hPow (β := ℕ)
          (norm (E := ℤ_[p])
            (((Polynomial.aeval a : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p])
                ((Polynomial.derivative : (a : Polynomial R) → Polynomial R) F) :
              ℤ_[p]))
          2 *
        T_gen p F a ^ HPow.hPow (α := ℕ) 2 n)
      2 /
    HPow.hPow (β := ℕ)
      (norm (E := ℤ_[p])
        (((Polynomial.aeval a : Polynomial R →ₐ[R] ℤ_[p]) : (a : Polynomial R) → ℤ_[p])
            ((Polynomial.derivative : (a : Polynomial R) → Polynomial R) F) :
          ℤ_[p]))
      2
Tactic: calc
    ‖F.aeval z'‖ = ‖q‖ * ‖z1‖ ^ 2 := by simp [heq]
    _ ≤ 1 * ‖z1‖ ^ 2 := by gcongr; apply PadicInt.norm_le_one
    _ = ‖F.aeval z‖ ^ 2 / ‖F.derivative.aeval a‖ ^ 2 := by simp [hzeq, hz.1, div_pow]
    _ ≤ (‖F.derivative.aeval a‖ ^ 2 * T ^ 2 ^ n) ^ 2 / ‖F.derivative.aeval a‖ ^ 2 := by
      gcongr
      exact hz.2
    _ = (‖F.derivative.aeval a‖ ^ 2) ^ 2 * (T ^ 2 ^ n) ^ 2 / ‖F.derivative.aeval a‖ ^ 2 := by
      simp only [mul_pow]
    _ = ‖F.derivative.aeval a‖ ^ 2 * (T ^ 2 ^ n) ^ 2 := div_sq_cancel _ _
    _ = ‖F.derivative.aeval a‖ ^ 2 * T ^ 2 ^ (n + 1) := by rw [← pow_mul, pow_succ 2]