-- 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))))
j : (x y : ↥c.pt) → I
fjx : (x y : ↥c.pt) → j x y ⟶ i x
fjy : (x y : ↥c.pt) → j x y ⟶ i y
hj :
  ∀ (x y : ↥c.pt),
    Eq (α :=
      ToType (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier)
        ((D.obj (j x y)).presheaf.obj (Opposite.op (D.map (fjx x y) ⁻¹ᵁ U x ⊓ D.map (fjy x y) ⁻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 x y)) (U x)) :
            (a : (↑Γ(D.obj (i x), U x) : Type u)) → (↑Γ(D.obj (j x y), D.map (fjx x y) ⁻¹ᵁ U x) : Type u))
          (t x))
        (D.map (fjx x y) ⁻¹ᵁ U x ⊓ D.map (fjy x y) ⁻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 x y)) (U y)) :
            (a : (↑Γ(D.obj (i y), U y) : Type u)) → (↑Γ(D.obj (j x y), D.map (fjy x y) ⁻¹ᵁ U y) : Type u))
          (t y))
        (D.map (fjx x y) ⁻¹ᵁ U x ⊓ D.map (fjy x y) ⁻¹ᵁ U y) ⋯)
σ : Finset ↥c.pt
hσ : Eq.{u + 1} (α := Set ↥c.pt) (⋃ x ∈ σ, (c.π.app (i x) ⁻¹ᵁ U x).carrier) ⊤
⊢ ∃ (i : I) (t : (↑Γ(D.obj i, ⊤) : Type u)),
    s =
      (ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
            CommRingCat.carrier) (Scheme.Hom.appTop (c.π.app i)) :
          (a : (↑Γ(D.obj i, ⊤) : Type u)) → (↑Γ(((Functor.const I).obj c.pt).obj i, ⊤) : Type u))
        t
have choose : ∀ (σi : (x : ↥c.pt) → ↥c.pt) (hσiσ : ∀ (x : ↥c.pt), σi x ∈ σ)
  (hσi :
    ∀ (x : ↥c.pt),
      Membership.mem (γ := Set ↥c.pt)
        (TopologicalSpace.Opens.carrier (c.π.app (i (σi x)) ⁻¹ᵁ U (σi x)) :
          Set ↥(Functor.obj (Functor.obj (Functor.const I) c.pt) (i (σi x))))
        x),
  ∃ (i : I) (t : (↑Γ(D.obj i, ⊤) : Type u)),
    s =
      (ConcreteCategory.hom (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
            CommRingCat.carrier) (Scheme.Hom.appTop (c.π.app i)) :
          (a : (↑Γ(D.obj i, ⊤) : Type u)) → (↑Γ(((Functor.const I).obj c.pt).obj i, ⊤) : Type u))
        t
Tactic: choose σi hσiσ hσi using fun x ↦ Set.mem_iUnion₂.mp (hσ.ge (Set.mem_univ x))