-- Lean 4 proof step from Mathlib.AlgebraicGeometry.ZariskisMainTheorem (goal)
Goal:
X Y : Scheme
f : X ⟶ Y
inst : LocallyOfFiniteType f
inst_1 : IsSeparated f
inst_2 : QuasiCompact f
x : ↥X
hx : QuasiFiniteAt f x
T : Scheme
fT : T ⟶ Y
left : Etale fT
u : ↥T
hu : Eq (α := ↥Y) (fT u : ↥Y) (f x : ↥Y)
V W : (pullback f fT).Opens
v : ↥V
hVW : IsCompl V W
left_1 : IsFinite (V.ι ≫ pullback.snd f fT)
hv₂ : (pullback.fst f fT) (↑v : ↥(pullback f fT)) = x
⊢ ∃ (V : (normalization f).Opens),
    Membership.mem (α := ↥(normalization f)) V ((toNormalization f) x : ↥(normalization f)) ∧
      IsIso (X := (↑(toNormalization f ⁻¹ᵁ V) : Scheme)) (toNormalization f ∣_ V)
have obtain : ∀ (U : (normalization (X := pullback f fT) (pullback.snd f fT)).Opens)
  (hU :
    Membership.mem (α := ↥(normalization (X := pullback f fT) (pullback.snd f fT))) U
      ((toNormalization (pullback.snd f fT)) (↑v : ↥(pullback f fT)) :
        ↥(normalization (X := pullback f fT) (pullback.snd f fT)))),
  IsIso (X := (↑(toNormalization (pullback.snd f fT) ⁻¹ᵁ U) : Scheme)) (toNormalization (pullback.snd f fT) ∣_ U) →
    ∃ (V : (normalization f).Opens),
      Membership.mem (α := ↥(normalization f)) V ((toNormalization f) x : ↥(normalization f)) ∧
        IsIso (X := (↑(toNormalization f ⁻¹ᵁ V) : Scheme)) (toNormalization f ∣_ V)
Tactic: obtain ⟨U, hU, _⟩ : ∃ U, (pullback.snd f fT).toNormalization v.1 ∈ U ∧
      IsIso ((pullback.snd f fT).toNormalization ∣_ U) := by
    have hVW' : (W : Set ↑(pullback f fT)) = (↑V)ᶜ :=
      eq_compl_iff_isCompl.mpr (hVW.map TopologicalSpace.Opens.frameHom).symm
    have : IsClosedImmersion V.ι := .of_isPreimmersion _ (by simp [eq_compl_comm.mp hVW', W.isOpen])
    have : IsClosedImmersion W.ι := .of_isPreimmersion _ (by simpa [hVW'] using V.2)
    obtain ⟨H⟩ := nonempty_isColimit_binaryCofanMk_of_isCompl V.ι W.ι (by simpa)
    let e : (pullback.snd f fT).normalization ≅ V ⨿ (W.ι ≫ pullback.snd f fT).normalization :=
      (Scheme.Hom.normalizationCoprodIso (pullback.snd f fT) H).symm ≪≫
        coprod.mapIso (asIso (V.ι ≫ pullback.snd f fT).toNormalization).symm (.refl _)
    let ι : V.toScheme ⟶ V ⨿ (W.ι ≫ pullback.snd f fT).normalization := coprod.inl
    refine ⟨e.hom ⁻¹ᵁ ι.opensRange, ⟨v, ?_⟩, ?_⟩
    · rw [← V.ι_apply, ← Scheme.Hom.comp_apply, ← Scheme.Hom.comp_apply]
      congr 5
      rw [← Category.assoc, ← Iso.comp_inv_eq]
      simp [ι, e, Scheme.Hom.normalizationCoprodIso]
    rw [← isIso_comp_right_iff _ (e.hom ∣_ ι.opensRange),
      ← morphismRestrict_comp, ← isIso_comp_right_iff _ ι.isoOpensRange.inv]
    have Heq : (pullback.snd f fT).toNormalization ⁻¹ᵁ e.hom ⁻¹ᵁ Scheme.Hom.opensRange ι = V := by
      apply le_antisymm
      · rintro a ⟨b, hab⟩
        by_contra h
        lift a to W using hVW'.ge h
        replace hab : ι b = ((W.ι ≫ pullback.snd f fT).toNormalization ≫ coprod.inr) a := by
          have : W.ι ≫ (H.coconePointUniqueUpToIso (colimit.isColimit _)).hom = coprod.inr :=
            H.comp_coconePointUniqueUpToIso_hom _ ⟨.right⟩
          simp only [← W.ι_apply, ← Scheme.Hom.comp_apply, Category.assoc, e] at hab
          simpa [-Scheme.Hom.comp_base, Scheme.Hom.normalizationCoprodIso,
            reassoc_of% this] using hab
        exact Set.disjoint_iff_forall_ne.mp
          (isCompl_range_inl_inr V (W.ι ≫ pullback.snd f fT).normalization).1 ⟨_, rfl⟩ ⟨_, rfl⟩ hab
      · rw [← Scheme.Hom.inv_image, ← SetLike.coe_subset_coe]
        simpa [← Scheme.Hom.opensRange_comp, ι, e, Scheme.Hom.normalizationCoprodIso,
          Set.range_comp] using Set.subset_preimage_image _ _
    convert! (inferInstance : IsIso (Scheme.isoOfEq _ Heq).hom)
    rw [Iso.comp_inv_eq, ← Iso.inv_comp_eq, ← cancel_mono (Scheme.Opens.ι _)]
    have : V.ι ≫ (H.coconePointUniqueUpToIso (colimit.isColimit _)).hom = coprod.inl :=
      H.comp_coconePointUniqueUpToIso_hom _ ⟨.left⟩
    simp [e, Scheme.Hom.isoOpensRange, Scheme.Hom.normalizationCoprodIso, reassoc_of% this, ι]