Module: Mathlib.AlgebraicGeometry.Morphisms.SurjectiveOnStalks

Goal state before:
pp: X Y S : Scheme
f : X ⟶ S
g : Y ⟶ S
inst✝ : SurjectiveOnStalks g
L : ↥(pullback f g) → ↥X × ↥Y := fun x ↦ ((pullback.fst f g) x, (pullback.snd f g) x)
H :
  ∀ (R A B : CommRingCat) (f' : Spec A ⟶ Spec R) (g' : Spec B ⟶ Spec R) (iX : Spec A ⟶ X) (iY : Spec B ⟶ Y)
    (iS : Spec R ⟶ S) (e₁ : f' ≫ iS = iX ≫ f) (e₂ : g' ≫ iS = iY ≫ g),
    IsOpenImmersion iX →
      IsOpenImmersion iY → IsOpenImmersion iS → IsEmbedding (L ∘ ⇑(pullback.map f' g' f g iX iY iS e₁ e₂))
𝒰 : S.OpenCover := S.affineOpenCover.openCover
𝒱 : (i : (Precoverage.ZeroHypercover.pullback₁ f 𝒰).I₀) → ((Precoverage.ZeroHypercover.pullback₁ f 𝒰).X i).OpenCover :=
  fun i ↦ ((Precoverage.ZeroHypercover.pullback₁ f 𝒰).X i).affineOpenCover.openCover
𝒲 : (i : (Precoverage.ZeroHypercover.pullback₁ g 𝒰).I₀) → ((Precoverage.ZeroHypercover.pullback₁ g 𝒰).X i).OpenCover :=
  fun i ↦ ((Precoverage.ZeroHypercover.pullback₁ g 𝒰).X i).affineOpenCover.openCover
U : (i : (Precoverage.ZeroHypercover.pullback₁ f 𝒰).I₀) × (𝒱 i).I₀ × (𝒲 i).I₀ → TopologicalSpace.Opens (↥X × ↥Y) :=
  fun ijk ↦
    {
      carrier :=
        {P |
          P.1 ∈ Scheme.Hom.opensRange ((𝒱 ijk.fst).f ijk.snd.1 ≫ (Precoverage.ZeroHypercover.pullback₁ f 𝒰).f ijk.fst) ∧
            P.2 ∈
              Scheme.Hom.opensRange ((𝒲 ijk.fst).f ijk.snd.2 ≫ (Precoverage.ZeroHypercover.pullback₁ g 𝒰).f ijk.fst)},
      is_open' := ⋯ }
z : ↥(pullback f g)
is : 𝒰.I₀
s : ↥(𝒰.X is)
hsx : (𝒰.f is) s = f ((pullback.fst f g) z)
hsy : (𝒰.f is) s = g ((pullback.snd f g) z)
⊢ ∃ a a_1 b, L z ∈ U ⟨a, (a_1, b)⟩

Tactic:
obtain ⟨x : (𝒰.pullback₁ f).X is, hx⟩ :=
      Scheme.IsJointlySurjectivePreserving.exists_preimage_fst_triplet_of_prop
        (P := @IsOpenImmersion) inferInstance _ _ hsx.symm

Goal state after:
X Y S : Scheme
f : X ⟶ S
g : Y ⟶ S
inst✝ : SurjectiveOnStalks g
L : ↥(pullback f g) → ↥X × ↥Y := fun x ↦ ((pullback.fst f g) x, (pullback.snd f g) x)
H :
  ∀ (R A B : CommRingCat) (f' : Spec A ⟶ Spec R) (g' : Spec B ⟶ Spec R) (iX : Spec A ⟶ X) (iY : Spec B ⟶ Y)
    (iS : Spec R ⟶ S) (e₁ : f' ≫ iS = iX ≫ f) (e₂ : g' ≫ iS = iY ≫ g),
    IsOpenImmersion iX →
      IsOpenImmersion iY → IsOpenImmersion iS → IsEmbedding (L ∘ ⇑(pullback.map f' g' f g iX iY iS e₁ e₂))
𝒰 : S.OpenCover := S.affineOpenCover.openCover
𝒱 : (i : (Precoverage.ZeroHypercover.pullback₁ f 𝒰).I₀) → ((Precoverage.ZeroHypercover.pullback₁ f 𝒰).X i).OpenCover :=
  fun i ↦ ((Precoverage.ZeroHypercover.pullback₁ f 𝒰).X i).affineOpenCover.openCover
𝒲 : (i : (Precoverage.ZeroHypercover.pullback₁ g 𝒰).I₀) → ((Precoverage.ZeroHypercover.pullback₁ g 𝒰).X i).OpenCover :=
  fun i ↦ ((Precoverage.ZeroHypercover.pullback₁ g 𝒰).X i).affineOpenCover.openCover
U : (i : (Precoverage.ZeroHypercover.pullback₁ f 𝒰).I₀) × (𝒱 i).I₀ × (𝒲 i).I₀ → TopologicalSpace.Opens (↥X × ↥Y) :=
  fun ijk ↦
    {
      carrier :=
        {P |
          P.1 ∈ Scheme.Hom.opensRange ((𝒱 ijk.fst).f ijk.snd.1 ≫ (Precoverage.ZeroHypercover.pullback₁ f 𝒰).f ijk.fst) ∧
            P.2 ∈
              Scheme.Hom.opensRange ((𝒲 ijk.fst).f ijk.snd.2 ≫ (Precoverage.ZeroHypercover.pullback₁ g 𝒰).f ijk.fst)},
      is_open' := ⋯ }
z : ↥(pullback f g)
is : 𝒰.I₀
s : ↥(𝒰.X is)
hsx : (𝒰.f is) s = f ((pullback.fst f g) z)
hsy : (𝒰.f is) s = g ((pullback.snd f g) z)
x : ↥((Precoverage.ZeroHypercover.pullback₁ f 𝒰).X is)
hx : (pullback.fst f (𝒰.f is)) x = (pullback.fst f g) z
⊢ ∃ a a_1 b, L z ∈ U ⟨a, (a_1, b)⟩