-- 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
⊢ Eq (α := (↑Γ(c.pt, c.π.app j ⁻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 (c.π.app j) (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(c.pt, c.π.app j ⁻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))))
          (β :=
          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))))
          ((ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                  CommRingCat.carrier) (X := (D.obj j).presheaf.obj (Opposite.op (D.map fjx ⁻¹ᵁ U x))) (Y :=
                  (D.obj j).presheaf.obj (Opposite.op (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)))
                  ((D.obj j).presheaf.map (Quiver.Hom.op (homOfLE ⋯))) :
                (a : (↑((D.obj j).presheaf.obj (Opposite.op (D.map fjx ⁻¹ᵁ U x))) : Type u)) →
                  (↑((D.obj j).presheaf.obj (Opposite.op (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 (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) :
                (↑Γ(D.obj j, D.map fjx ⁻¹ᵁ U x) : Type u)) :
            (↑((D.obj j).presheaf.obj (Opposite.op (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 := (D.obj j).presheaf.obj (Opposite.op (D.map fjy ⁻¹ᵁ U y))) (Y :=
                  (D.obj j).presheaf.obj (Opposite.op (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)))
                  ((D.obj j).presheaf.map (Quiver.Hom.op (homOfLE ⋯))) :
                (a : (↑((D.obj j).presheaf.obj (Opposite.op (D.map fjy ⁻¹ᵁ U y))) : Type u)) →
                  (↑((D.obj j).presheaf.obj (Opposite.op (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 (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) :
                (↑Γ(D.obj j, D.map fjy ⁻¹ᵁ U y) : Type u)) :
            (↑((D.obj j).presheaf.obj (Opposite.op (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y))) : Type u))) :
      (↑Γ(c.pt, c.π.app j ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) : Type u))
    (0 : (↑Γ(((Functor.const I).obj c.pt).obj j, c.π.app j ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) : Type u))
have simp : Eq (α := (↑Γ(c.pt, c.π.app j ⁻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) (Y := Γ(c.pt, c.π.app j ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)))
          (Scheme.Hom.appLE (c.π.app (i x)) (U x) (c.π.app j ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) sorry) :
        (a : (↑Γ(D.obj (i x), U x) : Type u)) →
          (↑Γ(c.pt, c.π.app j ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) : Type u))
      (t x) :
    (↑Γ(c.pt, c.π.app j ⁻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) (Y := Γ(c.pt, c.π.app j ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)))
          (Scheme.Hom.appLE (c.π.app (i y)) (U y) (c.π.app j ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) sorry) :
        (a : (↑Γ(D.obj (i y), U y) : Type u)) →
          (↑Γ(c.pt, c.π.app j ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) : Type u))
      (t y) :
    (↑Γ(c.pt, c.π.app j ⁻¹ᵁ (D.map fjx ⁻¹ᵁ U x ⊓ D.map fjy ⁻¹ᵁ U y)) : Type u))
Tactic: 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]