Module: Mathlib.AlgebraicGeometry.Morphisms.UniversallyInjective

Goal state before:
pp: case e_a
X Y : Scheme
f : X ⟶ Y
tfae_1_iff_4 : UniversallyInjective f ↔ Surjective (pullback.diagonal f)
tfae_3_to_2 :
  (Injective ⇑f ∧ ∀ (x : ↥X), (CommRingCat.Hom.hom (Scheme.Hom.residueFieldMap f x)).IsPurelyInseparable) →
    ∀ (K : Type u) [inst : Field K], Injective fun g ↦ g ≫ f
tfae_2_to_4 : (∀ (K : Type u) [inst : Field K], Injective fun g ↦ g ≫ f) → Surjective (pullback.diagonal f)
hf : Function.Surjective ⇑(pullback.diagonal f)
this : UniversallyInjective f
x : ↥X
algInst✝ : Algebra ↑(Y.residueField (f x)) ↑(X.residueField x) :=
  (CommRingCat.Hom.hom (Scheme.Hom.residueFieldMap f x)).toAlgebra
σ₁ σ₂ : Field.Emb ↑(Y.residueField (f x)) ↑(X.residueField x)
g₁ : Spec (CommRingCat.of (AlgebraicClosure ↑(X.residueField x))) ⟶ X :=
  (Scheme.SpecToEquivOfField (AlgebraicClosure ↑(X.residueField x)) X).symm ⟨x, CommRingCat.ofHom σ₁.toRingHom⟩
g₂ : Spec (CommRingCat.of (AlgebraicClosure ↑(X.residueField x))) ⟶ X :=
  (Scheme.SpecToEquivOfField (AlgebraicClosure ↑(X.residueField x)) X).symm ⟨x, CommRingCat.ofHom σ₂.toRingHom⟩
q : Spec (CommRingCat.of (AlgebraicClosure ↑(X.residueField x))) ⟶ pullback f f := pullback.lift g₁ g₂ ⋯
q_fst : q ≫ pullback.fst f f = g₁
q_snd : q ≫ pullback.snd f f = g₂
u : ↥X
hu : (pullback.diagonal f) u = q (IsLocalRing.closedPoint ↑(CommRingCat.of (AlgebraicClosure ↑(X.residueField x))))
hux : u = x
⊢ Scheme.Hom.residueFieldMap (pullback.fst f f) (q (IsLocalRing.closedPoint (AlgebraicClosure ↑(X.residueField x)))) =
    (Scheme.residueFieldCongr ⋯).hom ≫
      Scheme.Hom.residueFieldMap (pullback.snd f f) (q (IsLocalRing.closedPoint (AlgebraicClosure ↑(X.residueField x))))

Tactic:
rw [← cancel_mono (Scheme.residueFieldCongr (hux ▸ hu).symm).hom]

Goal state after:
case e_a
X Y : Scheme
f : X ⟶ Y
tfae_1_iff_4 : UniversallyInjective f ↔ Surjective (pullback.diagonal f)
tfae_3_to_2 :
  (Injective ⇑f ∧ ∀ (x : ↥X), (CommRingCat.Hom.hom (Scheme.Hom.residueFieldMap f x)).IsPurelyInseparable) →
    ∀ (K : Type u) [inst : Field K], Injective fun g ↦ g ≫ f
tfae_2_to_4 : (∀ (K : Type u) [inst : Field K], Injective fun g ↦ g ≫ f) → Surjective (pullback.diagonal f)
hf : Function.Surjective ⇑(pullback.diagonal f)
this : UniversallyInjective f
x : ↥X
algInst✝ : Algebra ↑(Y.residueField (f x)) ↑(X.residueField x) :=
  (CommRingCat.Hom.hom (Scheme.Hom.residueFieldMap f x)).toAlgebra
σ₁ σ₂ : Field.Emb ↑(Y.residueField (f x)) ↑(X.residueField x)
g₁ : Spec (CommRingCat.of (AlgebraicClosure ↑(X.residueField x))) ⟶ X :=
  (Scheme.SpecToEquivOfField (AlgebraicClosure ↑(X.residueField x)) X).symm ⟨x, CommRingCat.ofHom σ₁.toRingHom⟩
g₂ : Spec (CommRingCat.of (AlgebraicClosure ↑(X.residueField x))) ⟶ X :=
  (Scheme.SpecToEquivOfField (AlgebraicClosure ↑(X.residueField x)) X).symm ⟨x, CommRingCat.ofHom σ₂.toRingHom⟩
q : Spec (CommRingCat.of (AlgebraicClosure ↑(X.residueField x))) ⟶ pullback f f := pullback.lift g₁ g₂ ⋯
q_fst : q ≫ pullback.fst f f = g₁
q_snd : q ≫ pullback.snd f f = g₂
u : ↥X
hu : (pullback.diagonal f) u = q (IsLocalRing.closedPoint ↑(CommRingCat.of (AlgebraicClosure ↑(X.residueField x))))
hux : u = x
⊢ Scheme.Hom.residueFieldMap (pullback.fst f f) (q (IsLocalRing.closedPoint (AlgebraicClosure ↑(X.residueField x)))) ≫
      (Scheme.residueFieldCongr ⋯).hom =
    ((Scheme.residueFieldCongr ⋯).hom ≫
        Scheme.Hom.residueFieldMap (pullback.snd f f)
          (q (IsLocalRing.closedPoint (AlgebraicClosure ↑(X.residueField x))))) ≫
      (Scheme.residueFieldCongr ⋯).hom