-- Lean 4 proof step from Mathlib.AlgebraicGeometry.AffineTransitionLimit (goal)
Goal:
I : Type u
inst : Category.{u, u} I
D : I ⥤ Scheme
c : Cone D
hc : IsLimit c
inst_1 : IsCofiltered I
inst_2 : ∀ {i j : I} (f : i ⟶ j), IsAffineHom (D.map f)
s : (↑Γ(c.pt, ⊤) : Type u)
inst_3 : ∀ (i : I), CompactSpace.{u} ↥(D.obj i)
inst_4 : ∀ (i : I), QuasiSeparatedSpace.{u} ↥(D.obj i)
this : CompactSpace.{u} ↥c.pt
i : (x : ↥c.pt) → I
U : (x : ↥c.pt) → (D.obj (i x)).Opens
hU : ∀ (x : ↥c.pt), IsAffineOpen (U x)
hxU : ∀ (x : ↥c.pt), Membership.mem (α := ↥(D.obj (i x))) (U x) ((c.π.app (i x)) x : ↥(D.obj (i x)))
t : (x : ↥c.pt) → (↑Γ(D.obj (i x), U x) : Type u)
ht :
  ∀ (x : ↥c.pt),
    Eq (α := (↑Γ(c.pt, c.π.app (i x) ⁻¹ᵁ U x) : Type u))
      ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
              CommRingCat.carrier) (Y := Γ(c.pt, c.π.app (i x) ⁻¹ᵁ U x)) (Scheme.Hom.app (c.π.app (i x)) (U x)) :
            (a : (↑Γ(D.obj (i x), U x) : Type u)) → (↑Γ(c.pt, c.π.app (i x) ⁻¹ᵁ U x) : Type u))
          (t x) :
        (↑Γ(c.pt, c.π.app (i x) ⁻¹ᵁ U x) : Type u))
      (TopCat.Presheaf.restrictOpen (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
          CommRingCat.carrier) s (c.π.app (i x) ⁻¹ᵁ U x) ⋯ :
        ToType (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier)
          (c.pt.presheaf.obj (Opposite.op (c.π.app (i x) ⁻¹ᵁ U x))))
x y : ↥c.pt
j : I
fjx : j ⟶ i x
fjy : j ⟶ i y
⊢ ∃ (j : I) (fjx : j ⟶ i x) (fjy : j ⟶ i y),
    Eq (α :=
      ToType (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier)
        ((D.obj j).presheaf.obj (Opposite.op (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y))))
      (TopCat.Presheaf.restrictOpen (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
        CommRingCat.carrier)
        ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
              CommRingCat.carrier) (X := Γ(D.obj (i x), U x)) (Scheme.Hom.app (D.map fjx) (U x)) :
            (a : (↑Γ(D.obj (i x), U x) : Type u)) → (↑Γ(D.obj j, D.map fjx ⁻¹ᵁ U x) : Type u))
          (t x))
        (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y) ⋯)
      (TopCat.Presheaf.restrictOpen (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
        CommRingCat.carrier)
        ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
              CommRingCat.carrier) (X := Γ(D.obj (i y), U y)) (Scheme.Hom.app (D.map fjy) (U y)) :
            (a : (↑Γ(D.obj (i y), U y) : Type u)) → (↑Γ(D.obj j, D.map fjy ⁻¹ᵁ U y) : Type u))
          (t y))
        (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y) ⋯)
have obtain : ∀ (k : I) (fkj : k ⟶ j)
  (hk :
    Eq (α := (↑Γ(D.obj k, D.map fkj ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) : Type u))
      ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
              CommRingCat.carrier) (X := Γ(D.obj j, D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y))
              (Scheme.Hom.app (D.map fkj) (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) :
            (a : (↑Γ(D.obj j, D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y) : Type u)) →
              (↑Γ(D.obj k, D.map fkj ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) : Type u))
          (HSub.hSub (α :=
            ToType (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier)
              ((D.obj j).presheaf.obj (Opposite.op (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y))))
            (TopCat.Presheaf.restrictOpen (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
              CommRingCat.carrier)
              ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                    CommRingCat.carrier) (X := Γ(D.obj (i x), U x)) (Scheme.Hom.app (D.map fjx) (U x)) :
                  (a : (↑Γ(D.obj (i x), U x) : Type u)) → (↑Γ(D.obj j, D.map fjx ⁻¹ᵁ U x) : Type u))
                (t x))
              (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y) sorry)
            (TopCat.Presheaf.restrictOpen (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
              CommRingCat.carrier)
              ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                    CommRingCat.carrier) (X := Γ(D.obj (i y), U y)) (Scheme.Hom.app (D.map fjy) (U y)) :
                  (a : (↑Γ(D.obj (i y), U y) : Type u)) → (↑Γ(D.obj j, D.map fjy ⁻¹ᵁ U y) : Type u))
                (t y))
              (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y) sorry)) :
        (↑Γ(D.obj k, D.map fkj ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) : Type u))
      (0 : (↑Γ(D.obj k, D.map fkj ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) : Type u))),
  ∃ (j : I) (fjx : j ⟶ i x) (fjy : j ⟶ i y),
    Eq (α :=
      ToType (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier)
        ((D.obj j).presheaf.obj (Opposite.op (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y))))
      (TopCat.Presheaf.restrictOpen (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
        CommRingCat.carrier)
        ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
              CommRingCat.carrier) (X := Γ(D.obj (i x), U x)) (Scheme.Hom.app (D.map fjx) (U x)) :
            (a : (↑Γ(D.obj (i x), U x) : Type u)) → (↑Γ(D.obj j, D.map fjx ⁻¹ᵁ U x) : Type u))
          (t x))
        (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y) sorry)
      (TopCat.Presheaf.restrictOpen (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
        CommRingCat.carrier)
        ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
              CommRingCat.carrier) (X := Γ(D.obj (i y), U y)) (Scheme.Hom.app (D.map fjy) (U y)) :
            (a : (↑Γ(D.obj (i y), U y) : Type u)) → (↑Γ(D.obj j, D.map fjy ⁻¹ᵁ U y) : Type u))
          (t y))
        (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y) sorry)
Tactic: obtain ⟨k, fkj, hk⟩ := exists_app_map_eq_zero_of_isLimit D c hc
      (((hU x).preimage (D.map fjx)).isCompact.inter_of_isOpen
        ((hU y).preimage (D.map fjy)).isCompact ((U x).2.preimage (D.map fjx).continuous)
        ((U y).2.preimage (D.map fjy).continuous))
      ((D.map fjx).app (U x) (t x) |_ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y) -
        (D.map fjy).app (U y) (t y) |_ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) (by
      dsimp +instances [TopCat.Presheaf.restrictOpen, TopCat.Presheaf.restrict]
      simp only [map_sub, sub_eq_zero, ← ConcreteCategory.comp_apply,
        Scheme.Hom.app_eq_appLE, Scheme.Hom.appLE_map, Scheme.Hom.appLE_comp_appLE,
        Cone.w]
      simp_rw [Scheme.Hom.appLE, ConcreteCategory.comp_apply, ht, TopCat.Presheaf.restrictOpen,
        TopCat.Presheaf.restrict, ← ConcreteCategory.comp_apply, ← Functor.map_comp]
      rfl)