-- Lean 4 proof step from Mathlib.RingTheory.Henselian (goal)
Goal:
R : Type u_1
inst : CommRing R
I : Ideal R
inst_1 : IsAdicComplete I R
f : R[X]
h : f.Monic
a₀ : R
h₁ : Polynomial.eval a₀ f ∈ I
h₂ :
  IsUnit (M := R ⧸ I)
    ((Ideal.Quotient.mk I : (a : R) → R ⧸ I) (Polynomial.eval a₀ ((derivative : (a : R[X]) → R[X]) f)) : R ⧸ I)
let let₁ let₂ let₃ : ∃ (a : R), f.IsRoot a ∧ a - a₀ ∈ I
⊢ ∃ (a : R), f.IsRoot a ∧ a - a₀ ∈ I
have let₄ : let f' : R[X] := (derivative : (a : R[X]) → R[X]) f;
let c : ℕ → R := fun (n : ℕ) ↦
  Nat.recOn (motive := fun (x : ℕ) ↦ R) n a₀ fun (x : ℕ) (b : R) ↦ b - Polynomial.eval b f * (Polynomial.eval b f')⁻¹ʳ;
∀ (hc : ∀ (n : ℕ), c (n + 1) = c n - Polynomial.eval (c n) f * (Polynomial.eval (c n) f')⁻¹ʳ),
  ∃ (a : R), f.IsRoot a ∧ a - a₀ ∈ I
Tactic: classical
      let f' := derivative f
      -- we define a sequence `c n` by starting at `a₀` and then continually
      -- applying the function sending `b` to `b - f(b)/f'(b)` (Newton's method).
      -- Note that `f'.eval b` is a unit, because `b` has the same residue as `a₀` modulo `I`.
      let c : ℕ → R := fun n => Nat.recOn n a₀ fun _ b => b - f.eval b * (f'.eval b)⁻¹ʳ
      have hc : ∀ n, c (n + 1) = c n - f.eval (c n) * (f'.eval (c n))⁻¹ʳ := by
        intro n
        simp only [c]
      -- we now spend some time determining properties of the sequence `c : ℕ → R`
      -- `hc_mod`: for every `n`, we have `c n ≡ a₀ [SMOD I]`
      -- `hf'c`  : for every `n`, `f'.eval (c n)` is a unit
      -- `hfcI`  : for every `n`, `f.eval (c n)` is contained in `I ^ (n+1)`
      have hc_mod : ∀ n, c n ≡ a₀ [SMOD I] := by
        intro n
        induction n with
        | zero => rfl
        | succ n ih => ?_
        rw [hc, sub_eq_add_neg, ← add_zero a₀]
        refine ih.add ?_
        rw [SModEq.zero, Ideal.neg_mem_iff]
        refine I.mul_mem_right _ ?_
        rw [← SModEq.zero] at h₁ ⊢
        exact (ih.eval f).trans h₁
      have hf'c : ∀ n, IsUnit (f'.eval (c n)) := by
        intro n
        haveI := isLocalHom_of_le_jacobson_bot I (IsAdicComplete.le_jacobson_bot I)
        apply IsUnit.of_map (Ideal.Quotient.mk I)
        convert! h₂ using 1
        exact SModEq.def.mp ((hc_mod n).eval _)
      have hfcI : ∀ n, f.eval (c n) ∈ I ^ (n + 1) := by
        intro n
        induction n with
        | zero => simpa only [Nat.rec_zero, zero_add, pow_one] using h₁
        | succ n ih => ?_
        rw [← taylor_eval_sub (c n), hc, sub_eq_add_neg, sub_eq_add_neg,
          add_neg_cancel_comm]
        rw [eval_eq_sum, sum_over_range' _ _ _ (lt_add_of_pos_right _ zero_lt_two), ←
          Finset.sum_range_add_sum_Ico _ (Nat.le_add_left _ _)]
        swap
        · intro i
          rw [zero_mul]
        refine Ideal.add_mem _ ?_ ?_
        · rw [← one_add_one_eq_two, Finset.sum_range_succ, Finset.range_one, Finset.sum_singleton,
            taylor_coeff_zero, taylor_coeff_one, pow_zero, pow_one, mul_one, mul_neg,
            mul_left_comm, Ring.mul_inverse_cancel _ (hf'c n), mul_one, add_neg_cancel]
          exact Ideal.zero_mem _
        · refine Submodule.sum_mem _ ?_
          simp only [Finset.mem_Ico]
          rintro i ⟨h2i, _⟩
          have aux : n + 2 ≤ i * (n + 1) := by trans 2 * (n + 1) <;> nlinarith only [h2i]
          refine Ideal.mul_mem_left _ _ (Ideal.pow_le_pow_right aux ?_)
          rw [pow_mul']
          exact Ideal.pow_mem_pow ((Ideal.neg_mem_iff _).2 <| Ideal.mul_mem_right _ _ ih) _
      -- we are now in the position to show that `c : ℕ → R` is a Cauchy sequence
      have aux : ∀ m n, m ≤ n → c m ≡ c n [SMOD (I ^ m • ⊤ : Ideal R)] := by
        intro m n hmn
        rw [← Ideal.one_eq_top, Ideal.smul_eq_mul, mul_one]
        obtain ⟨k, rfl⟩ := Nat.exists_eq_add_of_le hmn
        clear hmn
        induction k with
        | zero => rw [add_zero]
        | succ k ih => ?_
        rw [← add_assoc, hc, ← add_zero (c m), sub_eq_add_neg]
        refine ih.add ?_
        symm
        rw [SModEq.zero, Ideal.neg_mem_iff]
        refine Ideal.mul_mem_right _ _ (Ideal.pow_le_pow_right ?_ (hfcI _))
        rw [add_assoc]
        exact le_self_add
      -- hence the sequence converges to some limit point `a`, which is the `a` we are looking for
      obtain ⟨a, ha⟩ := IsPrecomplete.prec' c (aux _ _)
      refine ⟨a, ?_, ?_⟩
      · show f.IsRoot a
        suffices ∀ n, f.eval a ≡ 0 [SMOD (I ^ n • ⊤ : Ideal R)] by exact IsHausdorff.haus' _ this
        intro n
        specialize ha n
        rw [← Ideal.one_eq_top, Ideal.smul_eq_mul, mul_one] at ha ⊢
        refine (ha.symm.eval f).trans ?_
        rw [SModEq.zero]
        exact Ideal.pow_le_pow_right le_self_add (hfcI _)
      · show a - a₀ ∈ I
        specialize ha (0 + 1)
        rw [hc, pow_one, ← Ideal.one_eq_top, Ideal.smul_eq_mul, mul_one, sub_eq_add_neg] at ha
        rw [← SModEq.sub_mem, ← add_zero a₀]
        refine ha.symm.trans (SModEq.rfl.add ?_)
        rw [SModEq.zero, Ideal.neg_mem_iff]
        exact Ideal.mul_mem_right _ _ h₁