-- Lean 4 proof step from Mathlib.CategoryTheory.Sites.Preserves (goal)
Goal:
C : Type u
inst : Category.{v, u} C
F : Functor.{v, w, u, w + 1} Cᵒᵖ (Type w)
α : Type u_1
inst_1 : Small.{w, u_1} α
X : α → C
c : Cofan.{u_1, v, u} X
hc : IsColimit c
inst_2 : (ofArrows X c.inj).HasPairwisePullbacks
inst_3 : PreservesLimit.{u_1, u_1, v, w, u, w + 1} (J := Discrete α) (Discrete.functor fun (x : α) ↦ op (X x)) F
this : HasCoproduct.{u_1, v, u} X
hi :
  ∀ (b : (fun (X : Type w) ↦ X) (∏ᶜ fun (b : α) ↦ F.obj (op (X b)))),
    ∃! a : (fun (X : Type w) ↦ X) (F.obj (piObj.{u_1, v, u} fun (x : α) ↦ op (X x))),
      ((ConcreteCategory.hom (FC := TypeCat.Fun)
                (F.map (Iso.inv (opCoproductIsoProduct' hc (productIsProduct fun (x : α) ↦ op (X x)))) ≫
                  Equalizer.Presieve.Arrows.forkMap F X c.inj) :
              TypeCat.Fun (F.obj (piObj.{u_1, v, u} fun (x : α) ↦ op (X x))) (∏ᶜ fun (b : α) ↦ F.obj (op (X b)))) :
            (a : (fun (X : Type w) ↦ X) (F.obj (∏ᶜ fun (x : α) ↦ op (X x)))) →
              (fun (X : Type w) ↦ X) (∏ᶜ fun (b : α) ↦ F.obj (op (X b))))
          a =
        b
b : Equalizer.Presieve.Arrows.FirstObj F X
h :
  Eq (α := (fun (X : Type w) ↦ X) (Equalizer.Presieve.Arrows.SecondObj F X c.inj))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (Equalizer.Presieve.Arrows.firstMap F X c.inj) :
          (a : (fun (X : Type w) ↦ X) (Equalizer.Presieve.Arrows.FirstObj F X)) →
            (fun (X : Type w) ↦ X) (Equalizer.Presieve.Arrows.SecondObj F X c.inj))
        b :
      (fun (X : Type w) ↦ X) (Equalizer.Presieve.Arrows.SecondObj F X c.inj))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (Equalizer.Presieve.Arrows.secondMap F X c.inj) :
          (a : (fun (X : Type w) ↦ X) (Equalizer.Presieve.Arrows.FirstObj F X)) →
            (fun (X : Type w) ↦ X) (Equalizer.Presieve.Arrows.SecondObj F X c.inj))
        b :
      (fun (X : Type w) ↦ X) (Equalizer.Presieve.Arrows.SecondObj F X c.inj))
⊢ ∃! x : F.obj (op c.pt),
    (ConcreteCategory.hom (FC := TypeCat.Fun) (Equalizer.Presieve.Arrows.forkMap F X c.inj) :
          (a : (fun (X : Type w) ↦ X) (F.obj (op c.pt))) →
            (fun (X : Type w) ↦ X) (Equalizer.Presieve.Arrows.FirstObj F X))
        x =
      b
have obtain : ∀ (t : F.obj (piObj.{u_1, v, u} fun (x : α) ↦ op (X x)))
  (ht₁ :
    ((ConcreteCategory.hom (FC := TypeCat.Fun)
              (F.map (Iso.inv (opCoproductIsoProduct' hc (productIsProduct fun (x : α) ↦ op (X x)))) ≫
                Equalizer.Presieve.Arrows.forkMap F X c.inj) :
            TypeCat.Fun (F.obj (piObj.{u_1, v, u} fun (x : α) ↦ op (X x))) (∏ᶜ fun (b : α) ↦ F.obj (op (X b)))) :
          (a : (fun (X : Type w) ↦ X) (F.obj (∏ᶜ fun (x : α) ↦ op (X x)))) →
            (fun (X : Type w) ↦ X) (∏ᶜ fun (b : α) ↦ F.obj (op (X b))))
        t =
      b)
  (ht₂ :
    ∀ (y : (fun (X : Type w) ↦ X) (F.obj (piObj.{u_1, v, u} fun (x : α) ↦ op (X x)))),
      (fun (a : (fun (X : Type w) ↦ X) (F.obj (piObj.{u_1, v, u} fun (x : α) ↦ op (X x)))) ↦
            ((ConcreteCategory.hom (FC := TypeCat.Fun)
                      (F.map (Iso.inv (opCoproductIsoProduct' hc (productIsProduct fun (x : α) ↦ op (X x)))) ≫
                        Equalizer.Presieve.Arrows.forkMap F X c.inj) :
                    TypeCat.Fun (F.obj (piObj.{u_1, v, u} fun (x : α) ↦ op (X x)))
                      (∏ᶜ fun (b : α) ↦ F.obj (op (X b)))) :
                  (a : (fun (X : Type w) ↦ X) (F.obj (∏ᶜ fun (x : α) ↦ op (X x)))) →
                    (fun (X : Type w) ↦ X) (∏ᶜ fun (b : α) ↦ F.obj (op (X b))))
                a =
              b)
          y →
        y = t),
  ∃! x : F.obj (op c.pt),
    (ConcreteCategory.hom (FC := TypeCat.Fun) (Equalizer.Presieve.Arrows.forkMap F X c.inj) :
          (a : (fun (X : Type w) ↦ X) (F.obj (op c.pt))) →
            (fun (X : Type w) ↦ X) (Equalizer.Presieve.Arrows.FirstObj F X))
        x =
      b
Tactic: obtain ⟨t, ht₁, ht₂⟩ := hi b