Module: Mathlib.AlgebraicGeometry.Morphisms.SurjectiveOnStalks

Goal state before:
pp: X Y S : Scheme
f : X ⟶ S
g : Y ⟶ S
inst✝ : SurjectiveOnStalks g
⊢ IsEmbedding fun x ↦ ((pullback.fst f g) x, (pullback.snd f g) x)

Tactic:
let L := (fun x ↦ (pullback.fst f g x, pullback.snd f g x))
  have H : ∀ R A B (f' : Spec A ⟶ Spec R) (g' : Spec B ⟶ Spec R) (iX : Spec A ⟶ X)
      (iY : Spec B ⟶ Y) (iS : Spec R ⟶ S) (e₁ e₂), IsOpenImmersion iX → IsOpenImmersion iY →
      IsOpenImmersion iS → IsEmbedding (L ∘ pullback.map f' g' f g iX iY iS e₁ e₂) := by
    intro R A B f' g' iX iY iS e₁ e₂ _ _ _
    have H : SurjectiveOnStalks g' :=
      have : SurjectiveOnStalks (g' ≫ iS) := e₂ ▸ inferInstance
      .of_comp _ iS
    obtain ⟨φ, rfl⟩ : ∃ φ, Spec.map φ = f' := ⟨_, Spec.map_preimage _⟩
    obtain ⟨ψ, rfl⟩ : ∃ ψ, Spec.map ψ = g' := ⟨_, Spec.map_preimage _⟩
    algebraize [φ.hom, ψ.hom]
    rw [HasRingHomProperty.Spec_iff (P := @SurjectiveOnStalks)] at H
    convert!
      ((iX.isOpenEmbedding.prodMap iY.isOpenEmbedding).isEmbedding.comp
            (PrimeSpectrum.isEmbedding_tensorProductTo_of_surjectiveOnStalks R A B H)).comp
        (Scheme.homeoOfIso (pullbackSpecIso R A B)).isEmbedding
    ext1 x
    obtain ⟨x, rfl⟩ := (Scheme.homeoOfIso (pullbackSpecIso R A B).symm).surjective x
    simp only [Scheme.homeoOfIso_apply, Function.comp_apply]
    ext
    · simp only [L, ← Scheme.Hom.comp_apply, pullback.lift_fst, Iso.symm_hom,
        Iso.inv_hom_id]
      erw [← Scheme.Hom.comp_apply, pullbackSpecIso_inv_fst_assoc]
      rfl
    · simp only [L, ← Scheme.Hom.comp_apply, pullback.lift_snd, Iso.symm_hom,
        Iso.inv_hom_id]
      erw [← Scheme.Hom.comp_apply, pullbackSpecIso_inv_snd_assoc]
      rfl
  let 𝒰 := S.affineOpenCover.openCover
  let 𝒱 (i) := ((𝒰.pullback₁ f).X i).affineOpenCover.openCover
  let 𝒲 (i) := ((𝒰.pullback₁ g).X i).affineOpenCover.openCover
  let U (ijk : Σ i, (𝒱 i).I₀ × (𝒲 i).I₀) : TopologicalSpace.Opens (X.carrier × Y) :=
    ⟨{ P | P.1 ∈ ((𝒱 ijk.1).f ijk.2.1 ≫ (𝒰.pullback₁ f).f ijk.1).opensRange ∧
          P.2 ∈ ((𝒲 ijk.1).f ijk.2.2 ≫ (𝒰.pullback₁ g).f ijk.1).opensRange },
      (continuous_fst.1 _ ((𝒱 ijk.1).f ijk.2.1 ≫
      (𝒰.pullback₁ f).f ijk.1).opensRange.2).inter (continuous_snd.1 _
      ((𝒲 ijk.1).f ijk.2.2 ≫ (𝒰.pullback₁ g).f ijk.1).opensRange.2)⟩
  have : Set.range L ⊆ (iSup U :) := by
    simp only [Precoverage.ZeroHypercover.pullback₁_toPreZeroHypercover,
      PreZeroHypercover.pullback₁_I₀, PreZeroHypercover.pullback₁_X, Set.range_subset_iff]
    intro z
    simp only [SetLike.mem_coe, TopologicalSpace.Opens.mem_iSup, Sigma.exists, Prod.exists]
    obtain ⟨is, s, hsx⟩ := 𝒰.exists_eq (f (pullback.fst f g z))
    have hsy : 𝒰.f is s = g (pullback.snd f g z) := by
      rwa [← Scheme.Hom.comp_apply, ← pullback.condition, Scheme.Hom.comp_apply]
    obtain ⟨x : (𝒰.pullback₁ f).X is, hx⟩ :=
      Scheme.IsJointlySurjectivePreserving.exists_preimage_fst_triplet_of_prop
        (P := @IsOpenImmersion) inferInstance _ _ hsx.symm
    obtain ⟨y : (𝒰.pullback₁ g).X is, hy⟩ :=
      Scheme.IsJointlySurjectivePreserving.exists_preimage_fst_triplet_of_prop
        (P := @IsOpenImmersion) inferInstance _ _ hsy.symm
    obtain ⟨ix, x, rfl⟩ := (𝒱 is).exists_eq x
    obtain ⟨iy, y, rfl⟩ := (𝒲 is).exists_eq y
    refine ⟨is, ix, iy, ⟨x, hx⟩, ⟨y, hy⟩⟩
  let 𝓤 := (Scheme.Pullback.openCoverOfBase 𝒰 f g).bind
    (fun i ↦ Scheme.Pullback.openCoverOfLeftRight (𝒱 i) (𝒲 i) _ _)
  refine isEmbedding_of_iSup_eq_top_of_preimage_subset_range _ ?_ U this _ (𝓤.f ·)
    (fun i ↦ (𝓤.f i).continuous) ?_ ?_
  · fun_prop
  · rintro i x ⟨⟨x₁, hx₁⟩, ⟨x₂, hx₂⟩⟩
    obtain ⟨x₁', hx₁'⟩ :=
      Scheme.IsJointlySurjectivePreserving.exists_preimage_fst_triplet_of_prop
        (P := @IsOpenImmersion) inferInstance _ _ hx₁.symm
    obtain ⟨x₂', hx₂'⟩ :=
      Scheme.IsJointlySurjectivePreserving.exists_preimage_fst_triplet_of_prop
        (P := @IsOpenImmersion) inferInstance _ _ hx₂.symm
    obtain ⟨z, hz⟩ :=
      Scheme.IsJointlySurjectivePreserving.exists_preimage_fst_triplet_of_prop
        (P := @IsOpenImmersion) inferInstance _ _ (hx₁'.trans hx₂'.symm)
    refine ⟨(pullbackFstFstIso _ _ _ _ _ _ (𝒰.f i.1) ?_ ?_).hom z, ?_⟩
    · simp [pullback.condition]
    · simp [pullback.condition]
    · dsimp only
      rw [← hx₁', ← hz, ← Scheme.Hom.comp_apply]
      erw [← Scheme.Hom.comp_apply]
      congr 5
      apply pullback.hom_ext <;> simp [𝓤, ← pullback.condition, ← pullback.condition_assoc]
  · intro i
    have := H (S.affineOpenCover.X i.1) (((𝒰.pullback₁ f).X i.1).affineOpenCover.X i.2.1)
        (((𝒰.pullback₁ g).X i.1).affineOpenCover.X i.2.2)
        ((𝒱 i.1).f i.2.1 ≫ 𝒰.pullbackHom f i.1)
        ((𝒲 i.1).f i.2.2 ≫ 𝒰.pullbackHom g i.1)
        ((𝒱 i.1).f i.2.1 ≫ (𝒰.pullback₁ f).f i.1)
        ((𝒲 i.1).f i.2.2 ≫ (𝒰.pullback₁ g).f i.1)
        (𝒰.f i.1) (by simp [pullback.condition]) (by simp [pullback.condition])
        inferInstance inferInstance inferInstance
    convert! this using 7
    apply pullback.hom_ext <;>
      simp [𝓤, Scheme.Cover.pullbackHom]

Goal state after: