-- Lean 4 proof step from Mathlib.RingTheory.Etale.QuasiFinite (goal)
Goal:
R : Type u_2
S : Type u_3
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
inst_5 : q.LiesOver p
inst_6 : FiniteType R S
inst_7 : QuasiFiniteAt R q
s₁ : S
hs₁q : s₁ ∉ q
hs₁ : ∀ (q' : Ideal S), q'.IsPrime → q' ≠ q → q'.LiesOver p → s₁ ∈ q'
s₂ : ↥(integralClosure R S)
hs₂q : (↑s₂ : S) ∉ q
hs₂ :
  Function.Bijective (α := Localization.Away s₂) (β :=
    Localization.Away (M := S)
      ((AlgHom.toRingHom (Subalgebra.val (integralClosure R S)) : (a : ↥(integralClosure R S)) → S) s₂ : S))
    (⇑(Localization.awayMap (AlgHom.toRingHom (Subalgebra.val (integralClosure R S))) s₂) :
      (a : Localization.Away s₂) →
        Localization.Away (((integralClosure R S).val.toRingHom : (a : ↥(integralClosure R S)) → S) s₂))
s₃ : Localization.Away s₂
hs₃ :
  Eq (α :=
    Localization.Away (M := S)
      ((AlgHom.toRingHom (Subalgebra.val (integralClosure R S)) : (a : ↥(integralClosure R S)) → S) s₂ : S))
    ((Localization.awayMap (AlgHom.toRingHom (Subalgebra.val (integralClosure R S))) s₂ :
          (a : Localization.Away s₂) →
            Localization.Away (((integralClosure R S).val.toRingHom : (a : ↥(integralClosure R S)) → S) s₂))
        s₃ :
      Localization.Away (M := S)
        ((AlgHom.toRingHom (Subalgebra.val (integralClosure R S)) : (a : ↥(integralClosure R S)) → S) s₂ : S))
    ((algebraMap S
            (Localization.Away (M := S)
              ((AlgHom.toRingHom (Subalgebra.val (integralClosure R S)) : (a : ↥(integralClosure R S)) → S) s₂ : S)) :
          (a : S) → Localization.Away (((integralClosure R S).val.toRingHom : (a : ↥(integralClosure R S)) → S) s₂))
        s₁ :
      Localization.Away (M := S)
        ((AlgHom.toRingHom (Subalgebra.val (integralClosure R S)) : (a : ↥(integralClosure R S)) → S) s₂ : S))
⊢ ∃ s ∉ q,
    ∃ (hs : IsIntegral R s),
      (∀ (q' : Ideal S), q'.IsPrime → q' ≠ q → q'.LiesOver p → s ∈ q') ∧
        ∀ (q' : Ideal ↥(integralClosure R S)),
          q'.IsPrime →
            q' ≠ Ideal.under (↥(integralClosure R S)) q →
              q'.LiesOver p →
                Subtype.mk (p := fun (x : S) ↦ Membership.mem (γ := Subalgebra R S) (integralClosure R S) x) s hs ∈ q'
have obtain : ∀ (s₃ : ↥(integralClosure R S)) (n : ℕ)
  (hs₃ :
    Eq (α :=
      Localization.Away (M := S)
        ((AlgHom.toRingHom (Subalgebra.val (integralClosure R S)) : (a : ↥(integralClosure R S)) → S) s₂ : S))
      ((Localization.awayMap (AlgHom.toRingHom (Subalgebra.val (integralClosure R S))) s₂ :
            (a : Localization.Away s₂) →
              Localization.Away (((integralClosure R S).val.toRingHom : (a : ↥(integralClosure R S)) → S) s₂))
          (IsLocalization.mk' (Localization.Away s₂) s₃
            (Subtype.mk (p := fun (x : ↥(integralClosure R S)) ↦
              Membership.mem (γ := Submonoid ↥(integralClosure R S)) (Submonoid.powers s₂) x) ((fun (x : ℕ) ↦ s₂ ^ x) n)
              sorry)) :
        Localization.Away (M := S)
          ((AlgHom.toRingHom (Subalgebra.val (integralClosure R S)) : (a : ↥(integralClosure R S)) → S) s₂ : S))
      ((algebraMap S
              (Localization.Away (M := S)
                ((AlgHom.toRingHom (Subalgebra.val (integralClosure R S)) : (a : ↥(integralClosure R S)) → S) s₂ : S)) :
            (a : S) → Localization.Away (((integralClosure R S).val.toRingHom : (a : ↥(integralClosure R S)) → S) s₂))
          s₁ :
        Localization.Away (M := S)
          ((AlgHom.toRingHom (Subalgebra.val (integralClosure R S)) : (a : ↥(integralClosure R S)) → S) s₂ : S))),
  ∃ s ∉ q,
    ∃ (hs : IsIntegral R s),
      (∀ (q' : Ideal S), q'.IsPrime → q' ≠ q → q'.LiesOver p → s ∈ q') ∧
        ∀ (q' : Ideal ↥(integralClosure R S)),
          q'.IsPrime →
            q' ≠ Ideal.under (↥(integralClosure R S)) q →
              q'.LiesOver p →
                Subtype.mk (p := fun (x : S) ↦ Membership.mem (γ := Subalgebra R S) (integralClosure R S) x) s hs ∈ q'
Tactic: obtain ⟨s₃, ⟨_, n, rfl⟩, rfl⟩ := IsLocalization.exists_mk'_eq (.powers s₂) s₃
Cleared hypotheses: s₃, hs₃