-- Lean 4 proof step from Mathlib.RingTheory.Unramified.LocalRing (goal)
Goal:
R : Type u_1
S : Type u_2
inst : CommRing R
inst_1 : CommRing S
inst_2 : Algebra R S
p : Ideal R
inst_3 : p.IsPrime
q : Ideal S
inst_4 : q.IsPrime
hRS : Ideal.FG (RingHom.ker (F := R →+* S) (algebraMap R S))
inst_5 : q.LiesOver p
H :
  Function.Injective.{u_1 + 1, u_2 + 1} (α := Localization.AtPrime p) (β := Localization.AtPrime q)
    (⇑(localRingHom p q (algebraMap R S) ⋯) : (a : Localization.AtPrime p) → Localization.AtPrime q)
obtain :
  ∀ (s : Finset R) (hs : Eq (α := Ideal R) (Ideal.span (↑s : Set R)) (RingHom.ker (F := R →+* S) (algebraMap R S))),
    ∃ r ∉ p,
      ∀ (r' : R),
        r ∣ r' →
          Function.Injective.{u_1 + 1, u_2 + 1} (α := Away r') (β :=
            Away (M := S) ((algebraMap R S : (a : R) → S) r' : S))
            (⇑(awayMap (algebraMap R S) r') : (a : Away r') → Away ((algebraMap R S : (a : R) → S) r'))
⊢ ∃ r ∉ p,
    ∀ (r' : R),
      r ∣ r' →
        Function.Injective.{u_1 + 1, u_2 + 1} (α := Away r') (β :=
          Away (M := S) ((algebraMap R S : (a : R) → S) r' : S))
          (⇑(awayMap (algebraMap R S) r') : (a : Away r') → Away ((algebraMap R S : (a : R) → S) r'))
have obtain₁ : ∀ (s : Finset R) (hs : Eq (α := Ideal R) (Ideal.span (↑s : Set R)) (RingHom.ker (F := R →+* S) (algebraMap R S)))
  (this :
    ∀ (x : ↥s),
      Eq (α := Localization.AtPrime p)
        ((algebraMap R (Localization.AtPrime p) : (a : R) → Localization.AtPrime p) (↑x : R) : Localization.AtPrime p)
        (0 : Localization.AtPrime p)),
  ∃ r ∉ p,
    ∀ (r' : R),
      r ∣ r' →
        Function.Injective.{u_1 + 1, u_2 + 1} (α := Away r') (β :=
          Away (M := S) ((algebraMap R S : (a : R) → S) r' : S))
          (⇑(awayMap (algebraMap R S) r') : (a : Away r') → Away ((algebraMap R S : (a : R) → S) r'))
Tactic: classical
  obtain ⟨s, hs⟩ := hRS
  have (x : s) : algebraMap R (Localization.AtPrime p) x.1 = 0 := by
    apply H
    simp [localRingHom_to_map, -FaithfulSMul.algebraMap_eq_zero_iff,
      show algebraMap R S _ = 0 from hs.le (Ideal.subset_span x.2)]
  choose m hm using fun x ↦ (IsLocalization.map_eq_zero_iff p.primeCompl _ _).mp (this x)
  have H : RingHom.ker (algebraMap R S) ≤ RingHom.ker
      (algebraMap R (Localization.Away (∏ i, m i).1)) := by
    rw [← hs, Ideal.span_le]
    intro x hxs
    refine (IsLocalization.map_eq_zero_iff (.powers (∏ i, m i).1) _ _).mpr ⟨⟨_, 1, rfl⟩, ?_⟩
    simp only [pow_one]
    rw [Fintype.prod_eq_mul_prod_compl ⟨x, hxs⟩, Submonoid.coe_mul, mul_assoc, mul_left_comm, hm,
      mul_zero]
  refine ⟨_, (∏ i : s, m i).2, ?_⟩
  rintro r' ⟨s, e⟩
  rw [injective_iff_map_eq_zero]
  intro x hx
  obtain ⟨x, _, rfl⟩ := IsLocalization.exists_mk'_eq (.powers r') x
  simp only [awayMap, IsLocalization.Away.map, IsLocalization.map_mk',
    IsLocalization.mk'_eq_zero_iff] at hx
  obtain ⟨⟨_, n, rfl⟩, hn⟩ := hx
  simp only [← map_pow, ← map_mul] at hn
  obtain ⟨⟨_, k, rfl⟩, hk⟩ := (IsLocalization.map_eq_zero_iff (.powers (∏ i, m i).1) _ _).mp (H hn)
  refine (IsLocalization.mk'_eq_zero_iff _ _).mpr ⟨⟨_, k + n, rfl⟩, ?_⟩
  dsimp only at hk ⊢
  rw [pow_add, mul_assoc, e, mul_pow, ← e, mul_assoc, mul_left_comm, hk, mul_zero]
Cleared hypotheses: hRS