Module: Mathlib.AlgebraicGeometry.Morphisms.Flat

Goal state before:
pp: case intro
X Y S T : Scheme
f : T ⟶ S
g : Y ⟶ X
iX : X ⟶ S
iY : Y ⟶ T
H : IsPullback g iY iX f
US : S.Opens
UT : T.Opens
UX : X.Opens
hUST : UT ≤ f ⁻¹ᵁ US
hUSX : UX ≤ iX ⁻¹ᵁ US
UY : Y.Opens
hUY : UY = g ⁻¹ᵁ UX ⊓ iY ⁻¹ᵁ UT
ι : Type u
inst✝ : Finite ι
VX : ι → X.Opens
hVU : iSup VX = UX
hV : ∀ (i : ι), IsIso (pushoutSection H hUST ⋯ ⋯)
hV' : ∀ (i j : ι), Mono (pushoutSection H hUST ⋯ ⋯)
hT : (CommRingCat.Hom.hom (Scheme.Hom.appLE f US UT hUST)).Flat
D : CategoryTheory.Pairwise ι ⥤ X.Opens := Pairwise.diagram VX
h : iSup D.obj = UX
c₀ : Cocone D := (colimit.cocone D).extend (eqToIso ⋯).hom
F : (CategoryTheory.Pairwise ι)ᵒᵖ ⥤ CommRingCat :=
  Under.lift (D.op ⋙ X.presheaf)
      ((Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ).map (Scheme.Hom.appLE iX US UX hUSX) ≫
        (Functor.mapCone X.presheaf c₀.op).π) ⋙
    Under.pushout (Scheme.Hom.appLE f US UT hUST) ⋙ Under.forget Γ(T, UT)
G : X.Opens ⥤ Y.Opens :=
  { obj := fun U ↦ g ⁻¹ᵁ U ⊓ iY ⁻¹ᵁ UT, map := fun {X_1 Y_1} h ↦ homOfLE ⋯, map_id := ⋯, map_comp := ⋯ }
αF : F ⟶ D.op ⋙ G.op ⋙ Y.presheaf :=
  { app := fun i ↦ (pushout.congrHom ⋯ ⋯).hom ≫ pushoutSection H hUST ⋯ ⋯, naturality := ⋯ }
c : Cone F :=
  (Under.pushout (Scheme.Hom.appLE f US UT hUST) ⋙ Under.forget Γ(T, UT)).mapCone
    (Under.liftCone (Functor.mapCone X.presheaf c₀.op) (Scheme.Hom.appLE iX US UX hUSX))
this : PreservesFiniteLimits (Under.pushout (Scheme.Hom.appLE f US UT hUST))
val✝ : Fintype ι
⊢ IsIso (pushoutSection H hUST hUSX hUY)

Tactic:
let hc : IsLimit c :=
    haveI HX := ((TopCat.Presheaf.isSheaf_iff_isSheafPreservesLimitPairwiseIntersections
      _).mp X.IsSheaf VX).preserves (c := c₀.op)
    haveI HX := (HX (IsColimit.extendIso _ (colimit.isColimit _)).op).some
    isLimitOfPreserves (Under.pushout _ ⋙ Under.forget _) (Under.isLimitLiftCone _ _ HX)

Goal state after:
case intro
X Y S T : Scheme
f : T ⟶ S
g : Y ⟶ X
iX : X ⟶ S
iY : Y ⟶ T
H : IsPullback g iY iX f
US : S.Opens
UT : T.Opens
UX : X.Opens
hUST : UT ≤ f ⁻¹ᵁ US
hUSX : UX ≤ iX ⁻¹ᵁ US
UY : Y.Opens
hUY : UY = g ⁻¹ᵁ UX ⊓ iY ⁻¹ᵁ UT
ι : Type u
inst✝ : Finite ι
VX : ι → X.Opens
hVU : iSup VX = UX
hV : ∀ (i : ι), IsIso (pushoutSection H hUST ⋯ ⋯)
hV' : ∀ (i j : ι), Mono (pushoutSection H hUST ⋯ ⋯)
hT : (CommRingCat.Hom.hom (Scheme.Hom.appLE f US UT hUST)).Flat
D : CategoryTheory.Pairwise ι ⥤ X.Opens := Pairwise.diagram VX
h : iSup D.obj = UX
c₀ : Cocone D := (colimit.cocone D).extend (eqToIso ⋯).hom
F : (CategoryTheory.Pairwise ι)ᵒᵖ ⥤ CommRingCat :=
  Under.lift (D.op ⋙ X.presheaf)
      ((Functor.const (CategoryTheory.Pairwise ι)ᵒᵖ).map (Scheme.Hom.appLE iX US UX hUSX) ≫
        (Functor.mapCone X.presheaf c₀.op).π) ⋙
    Under.pushout (Scheme.Hom.appLE f US UT hUST) ⋙ Under.forget Γ(T, UT)
G : X.Opens ⥤ Y.Opens :=
  { obj := fun U ↦ g ⁻¹ᵁ U ⊓ iY ⁻¹ᵁ UT, map := fun {X_1 Y_1} h ↦ homOfLE ⋯, map_id := ⋯, map_comp := ⋯ }
αF : F ⟶ D.op ⋙ G.op ⋙ Y.presheaf :=
  { app := fun i ↦ (pushout.congrHom ⋯ ⋯).hom ≫ pushoutSection H hUST ⋯ ⋯, naturality := ⋯ }
c : Cone F :=
  (Under.pushout (Scheme.Hom.appLE f US UT hUST) ⋙ Under.forget Γ(T, UT)).mapCone
    (Under.liftCone (Functor.mapCone X.presheaf c₀.op) (Scheme.Hom.appLE iX US UX hUSX))
this : PreservesFiniteLimits (Under.pushout (Scheme.Hom.appLE f US UT hUST))
val✝ : Fintype ι
hc : IsLimit c :=
  isLimitOfPreserves (Under.pushout (Scheme.Hom.appLE f US UT hUST) ⋙ Under.forget Γ(T, UT))
    (Under.isLimitLiftCone (Functor.mapCone X.presheaf c₀.op) (Scheme.Hom.appLE iX US UX hUSX) ⋯.some)
⊢ IsIso (pushoutSection H hUST hUSX hUY)