-- Mathlib/AlgebraicGeometry/Morphisms/RingHomProperties.lean
/-- If `P` holds for `f` over affine opens `U₂` of `Y` and `V₂` of `X` and `U₁` (resp. `V₁`) are
open affine neighborhoods of `x` (resp. `f.base x`), then `P` also holds for `f`
over some basic open of `U₁` (resp. `V₁`). -/
lemma exists_basicOpen_le_appLE_of_appLE_of_isAffine
    (hPa : StableUnderCompositionWithLocalizationAwayTarget P) (hPl : LocalizationAwayPreserves P)
    (x : X) (U₁ : Y.affineOpens) (U₂ : Y.affineOpens) (V₁ : X.affineOpens) (V₂ : X.affineOpens)
    (hx₁ : x ∈ V₁.1) (hx₂ : x ∈ V₂.1) (e₂ : V₂.1 ≤ f ⁻¹ᵁ U₂.1) (h₂ : P (f.appLE U₂ V₂ e₂).hom)
    (hfx₁ : f.base x ∈ U₁.1) :
    ∃ (r : Γ(Y, U₁)) (s : Γ(X, V₁)) (_ : x ∈ X.basicOpen s)
      (e : X.basicOpen s ≤ f ⁻¹ᵁ Y.basicOpen r),
        P (f.appLE (Y.basicOpen r) (X.basicOpen s) e).hom

Proof state:
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
X Y : Scheme
f : X ⟶ Y
hPa : StableUnderCompositionWithLocalizationAwayTarget fun {R S} [CommRing R] [CommRing S] => P
hPl : LocalizationAwayPreserves fun {R S} [CommRing R] [CommRing S] => P
x : ↑↑X.toPresheafedSpace
U₁ U₂ : ↑Y.affineOpens
V₁ V₂ : ↑X.affineOpens
hx₁ : x ∈ ↑V₁
hx₂ : x ∈ ↑V₂
e₂ : ↑V₂ ≤ f ⁻¹ᵁ ↑U₂
h₂ : P (CommRingCat.Hom.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂))
hfx₁ : (ConcreteCategory.hom f.base) x ∈ ↑U₁
⊢ ∃ r s,
    ∃ (_ : x ∈ X.basicOpen s) (e : X.basicOpen s ≤ f ⁻¹ᵁ Y.basicOpen r),
      P (CommRingCat.Hom.hom (Scheme.Hom.appLE f (Y.basicOpen r) (X.basicOpen s) e))

Tactic:
obtain ⟨r, r', hBrr', hBfx⟩ := exists_basicOpen_le_affine_inter U₁.2 U₂.2 (f.base x)
  ⟨hfx₁, e₂ hx₂⟩

Result:
case intro.intro.intro
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
X Y : Scheme
f : X ⟶ Y
hPa : StableUnderCompositionWithLocalizationAwayTarget fun {R S} [CommRing R] [CommRing S] => P
hPl : LocalizationAwayPreserves fun {R S} [CommRing R] [CommRing S] => P
x : ↑↑X.toPresheafedSpace
U₁ U₂ : ↑Y.affineOpens
V₁ V₂ : ↑X.affineOpens
hx₁ : x ∈ ↑V₁
hx₂ : x ∈ ↑V₂
e₂ : ↑V₂ ≤ f ⁻¹ᵁ ↑U₂
h₂ : P (CommRingCat.Hom.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂))
hfx₁ : (ConcreteCategory.hom f.base) x ∈ ↑U₁
r : ↑Γ(Y, ↑U₁)
r' : ↑Γ(Y, ↑U₂)
hBrr' : Y.basicOpen r = Y.basicOpen r'
hBfx : (ConcreteCategory.hom f.base) x ∈ Y.basicOpen r
⊢ ∃ r s,
    ∃ (_ : x ∈ X.basicOpen s) (e : X.basicOpen s ≤ f ⁻¹ᵁ Y.basicOpen r),
      P (CommRingCat.Hom.hom (Scheme.Hom.appLE f (Y.basicOpen r) (X.basicOpen s) e))

Proof state:
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
X Y : Scheme
f : X ⟶ Y
hPa : StableUnderCompositionWithLocalizationAwayTarget fun {R S} [CommRing R] [CommRing S] => P
hPl : LocalizationAwayPreserves fun {R S} [CommRing R] [CommRing S] => P
x : ↑↑X.toPresheafedSpace
U₁ U₂ : ↑Y.affineOpens
V₁ V₂ : ↑X.affineOpens
hx₁ : x ∈ ↑V₁
hx₂ : x ∈ ↑V₂
e₂ : ↑V₂ ≤ f ⁻¹ᵁ ↑U₂
h₂ : P (CommRingCat.Hom.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂))
hfx₁ : (ConcreteCategory.hom f.base) x ∈ ↑U₁
r : ↑Γ(Y, ↑U₁)
r' : ↑Γ(Y, ↑U₂)
hBrr' : Y.basicOpen r = Y.basicOpen r'
hBfx : (ConcreteCategory.hom f.base) x ∈ Y.basicOpen r
ha : IsAffineOpen (X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r'))
hxa : x ∈ X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r')
s : ↑Γ(X, ↑V₁)
s' : ↑Γ(X, X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r'))
hBss' : X.basicOpen s = X.basicOpen s'
hBx : x ∈ X.basicOpen s
this✝¹ :
  IsLocalization.Away ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r')
    ↑Γ(X, X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r'))
this✝ : IsLocalization.Away r' ↑Γ(Y, Y.basicOpen r')
this : IsLocalization.Away s' ↑Γ(X, X.basicOpen s')
ers : X.basicOpen s ≤ f ⁻¹ᵁ Y.basicOpen r
⊢ Scheme.Hom.app f (Y.basicOpen r') ≫ X.presheaf.map (homOfLE ⋯).op =
    Scheme.Hom.app f (Y.basicOpen r') ≫
      X.presheaf.map (homOfLE ⋯).op ≫
        CommRingCat.ofHom
          (algebraMap
            ↑(X.presheaf.toPrefunctor.1
                (Opposite.op
                  (X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.app f ↑U₂ ≫ X.presheaf.map (homOfLE e₂).op)) r'))))
            ↑(X.presheaf.toPrefunctor.1 (Opposite.op (X.basicOpen s'))))

Tactic:
congr

Result:
case e_a
P : {R S : Type u} → [inst : CommRing R] → [inst_1 : CommRing S] → (R →+* S) → Prop
X Y : Scheme
f : X ⟶ Y
hPa : StableUnderCompositionWithLocalizationAwayTarget fun {R S} [CommRing R] [CommRing S] => P
hPl : LocalizationAwayPreserves fun {R S} [CommRing R] [CommRing S] => P
x : ↑↑X.toPresheafedSpace
U₁ U₂ : ↑Y.affineOpens
V₁ V₂ : ↑X.affineOpens
hx₁ : x ∈ ↑V₁
hx₂ : x ∈ ↑V₂
e₂ : ↑V₂ ≤ f ⁻¹ᵁ ↑U₂
h₂ : P (CommRingCat.Hom.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂))
hfx₁ : (ConcreteCategory.hom f.base) x ∈ ↑U₁
r : ↑Γ(Y, ↑U₁)
r' : ↑Γ(Y, ↑U₂)
hBrr' : Y.basicOpen r = Y.basicOpen r'
hBfx : (ConcreteCategory.hom f.base) x ∈ Y.basicOpen r
ha : IsAffineOpen (X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r'))
hxa : x ∈ X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r')
s : ↑Γ(X, ↑V₁)
s' : ↑Γ(X, X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r'))
hBss' : X.basicOpen s = X.basicOpen s'
hBx : x ∈ X.basicOpen s
this✝¹ :
  IsLocalization.Away ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r')
    ↑Γ(X, X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.appLE f (↑U₂) (↑V₂) e₂)) r'))
this✝ : IsLocalization.Away r' ↑Γ(Y, Y.basicOpen r')
this : IsLocalization.Away s' ↑Γ(X, X.basicOpen s')
ers : X.basicOpen s ≤ f ⁻¹ᵁ Y.basicOpen r
⊢ X.presheaf.map (homOfLE ⋯).op =
    X.presheaf.map (homOfLE ⋯).op ≫
      CommRingCat.ofHom
        (algebraMap
          ↑(X.presheaf.toPrefunctor.1
              (Opposite.op
                (X.basicOpen ((ConcreteCategory.hom (Scheme.Hom.app f ↑U₂ ≫ X.presheaf.map (homOfLE e₂).op)) r'))))
          ↑(X.presheaf.toPrefunctor.1 (Opposite.op (X.basicOpen s'))))