Module: Mathlib.AlgebraicGeometry.ValuativeCriterion

Goal state before:
pp: X Y : Scheme
f : X ⟶ Y
H : Existence f
x' : ↥X
y : ↥Y
h : flip (fun x1 x2 ↦ x1 ⤳ x2) y (f x')
stalk_y_to_residue_x' : Y.presheaf.stalk y ⟶ X.residueField x' :=
  Y.presheaf.stalkSpecializes h ≫ Scheme.Hom.stalkMap f x' ≫ X.residue x'
A : ValuationSubring ↑(X.residueField x')
hA : ∀ (x : ↑(Y.presheaf.stalk y)), (CommRingCat.Hom.hom stalk_y_to_residue_x') x ∈ A.toSubring
hA_local : IsLocalHom ((CommRingCat.Hom.hom stalk_y_to_residue_x').codRestrict A.toSubring hA)
stalk_y_to_A : Y.presheaf.stalk y ⟶ CommRingCat.of ↥A :=
  CommRingCat.ofHom ((CommRingCat.Hom.hom stalk_y_to_residue_x').codRestrict A hA)
w :
  X.fromSpecResidueField x' ≫ f =
    Spec.map (CommRingCat.ofHom (algebraMap ↥A ↑(X.residueField x'))) ≫ Spec.map stalk_y_to_A ≫ Y.fromSpecStalk y
⊢ ∃ a', flip (fun x1 x2 ↦ x1 ⤳ x2) a' x' ∧ f a' = y

Tactic:
obtain ⟨l, hl₁, hl₂⟩ := (H { R := A, K := X.residueField x', commSq := ⟨w⟩, .. }).exists_lift

Goal state after:
X Y : Scheme
f : X ⟶ Y
H : Existence f
x' : ↥X
y : ↥Y
h : flip (fun x1 x2 ↦ x1 ⤳ x2) y (f x')
stalk_y_to_residue_x' : Y.presheaf.stalk y ⟶ X.residueField x' :=
  Y.presheaf.stalkSpecializes h ≫ Scheme.Hom.stalkMap f x' ≫ X.residue x'
A : ValuationSubring ↑(X.residueField x')
hA : ∀ (x : ↑(Y.presheaf.stalk y)), (CommRingCat.Hom.hom stalk_y_to_residue_x') x ∈ A.toSubring
hA_local : IsLocalHom ((CommRingCat.Hom.hom stalk_y_to_residue_x').codRestrict A.toSubring hA)
stalk_y_to_A : Y.presheaf.stalk y ⟶ CommRingCat.of ↥A :=
  CommRingCat.ofHom ((CommRingCat.Hom.hom stalk_y_to_residue_x').codRestrict A hA)
w :
  X.fromSpecResidueField x' ≫ f =
    Spec.map (CommRingCat.ofHom (algebraMap ↥A ↑(X.residueField x'))) ≫ Spec.map stalk_y_to_A ≫ Y.fromSpecStalk y
l :
  Spec
      (CommRingCat.of
        { R := ↥A, commRing := A.instCommRingSubtypeMem, domain := ⋯, valuationRing := ⋯, K := ↑(X.residueField x'),
            field := X.instFieldCarrierResidueField x', algebra := A.instAlgebraSubtypeMem, isFractionRing := ⋯,
            i₁ := X.fromSpecResidueField x', i₂ := Spec.map stalk_y_to_A ≫ Y.fromSpecStalk y, commSq := ⋯ }.R) ⟶
    X
hl₁ :
  Spec.map
        (CommRingCat.ofHom
          (algebraMap
            { R := ↥A, commRing := A.instCommRingSubtypeMem, domain := ⋯, valuationRing := ⋯, K := ↑(X.residueField x'),
                field := X.instFieldCarrierResidueField x', algebra := A.instAlgebraSubtypeMem, isFractionRing := ⋯,
                i₁ := X.fromSpecResidueField x', i₂ := Spec.map stalk_y_to_A ≫ Y.fromSpecStalk y, commSq := ⋯ }.R
            { R := ↥A, commRing := A.instCommRingSubtypeMem, domain := ⋯, valuationRing := ⋯, K := ↑(X.residueField x'),
                field := X.instFieldCarrierResidueField x', algebra := A.instAlgebraSubtypeMem, isFractionRing := ⋯,
                i₁ := X.fromSpecResidueField x', i₂ := Spec.map stalk_y_to_A ≫ Y.fromSpecStalk y, commSq := ⋯ }.K)) ≫
      l =
    { R := ↥A, commRing := A.instCommRingSubtypeMem, domain := ⋯, valuationRing := ⋯, K := ↑(X.residueField x'),
        field := X.instFieldCarrierResidueField x', algebra := A.instAlgebraSubtypeMem, isFractionRing := ⋯,
        i₁ := X.fromSpecResidueField x', i₂ := Spec.map stalk_y_to_A ≫ Y.fromSpecStalk y, commSq := ⋯ }.i₁
hl₂ :
  l ≫ f =
    { R := ↥A, commRing := A.instCommRingSubtypeMem, domain := ⋯, valuationRing := ⋯, K := ↑(X.residueField x'),
        field := X.instFieldCarrierResidueField x', algebra := A.instAlgebraSubtypeMem, isFractionRing := ⋯,
        i₁ := X.fromSpecResidueField x', i₂ := Spec.map stalk_y_to_A ≫ Y.fromSpecStalk y, commSq := ⋯ }.i₂
⊢ ∃ a', flip (fun x1 x2 ↦ x1 ⤳ x2) a' x' ∧ f a' = y