-- Lean 4 proof step from Mathlib.Order.Category.PartOrdEmb (goal)
Goal:
J : Type u
inst : SmallCategory J
inst_1 : IsFiltered J
F : J ⥤ PartOrdEmb
c :
  Cocone (F ⋙ forget PartOrdEmb (FC := fun (x1 x2 : PartOrdEmb.{u}) ↦ (↑x1 : Type u) ↪o (↑x2 : Type u)) (CC := carrier))
hc : IsColimit c
x y z : CoconePt hc
j : J
x₁ y₁ : (↑(F.obj j) : Type u)
hx₁ :
  (ConcreteCategory.hom (FC := TypeCat.Fun) (c.ι.app j) :
        (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj j)) →
          (fun (X : Type u) ↦ X) (((Functor.const J).obj c.pt).obj j))
      x₁ =
    x
hy₁ :
  (ConcreteCategory.hom (FC := TypeCat.Fun) (c.ι.app j) :
        (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj j)) →
          (fun (X : Type u) ↦ X) (((Functor.const J).obj c.pt).obj j))
      y₁ =
    y
hxy : x₁ ≤ y₁
k : J
y₂ z₁ : (↑(F.obj k) : Type u)
hy₂ :
  (ConcreteCategory.hom (FC := TypeCat.Fun) (c.ι.app k) :
        (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj k)) →
          (fun (X : Type u) ↦ X) (((Functor.const J).obj c.pt).obj k))
      y₂ =
    y
hz₁ :
  (ConcreteCategory.hom (FC := TypeCat.Fun) (c.ι.app k) :
        (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj k)) →
          (fun (X : Type u) ↦ X) (((Functor.const J).obj c.pt).obj k))
      z₁ =
    z
hyz : y₂ ≤ z₁
⊢ ∃ (j : J) (x' : (↑(F.obj j) : Type u)) (y' : (↑(F.obj j) : Type u)) (_ :
    (ConcreteCategory.hom (FC := TypeCat.Fun) (c.ι.app j) :
          (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj j)) →
            (fun (X : Type u) ↦ X) (((Functor.const J).obj c.pt).obj j))
        x' =
      x)
    (_ :
    (ConcreteCategory.hom (FC := TypeCat.Fun) (c.ι.app j) :
          (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj j)) →
            (fun (X : Type u) ↦ X) (((Functor.const J).obj c.pt).obj j))
        y' =
      z),
    x' ≤ y'
have obtain : ∀ (l : J) (a : j ⟶ l) (b : k ⟶ l)
  (h :
    Eq (α :=
      (fun (X : Type u) ↦ X)
        ((F ⋙
              forget PartOrdEmb (FC := fun (x1 x2 : PartOrdEmb.{u}) ↦ (↑x1 : Type u) ↪o (↑x2 : Type u)) (CC :=
                carrier)).obj
          l))
      ((ConcreteCategory.hom (FC := TypeCat.Fun)
              ((F ⋙
                    forget PartOrdEmb (FC := fun (x1 x2 : PartOrdEmb.{u}) ↦ (↑x1 : Type u) ↪o (↑x2 : Type u)) (CC :=
                      carrier)).map
                a) :
            (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj j)) →
              (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj l))
          y₁ :
        (fun (X : Type u) ↦ X)
          ((F ⋙
                forget PartOrdEmb (FC := fun (x1 x2 : PartOrdEmb.{u}) ↦ (↑x1 : Type u) ↪o (↑x2 : Type u)) (CC :=
                  carrier)).obj
            l))
      ((ConcreteCategory.hom (FC := TypeCat.Fun)
              ((F ⋙
                    forget PartOrdEmb (FC := fun (x1 x2 : PartOrdEmb.{u}) ↦ (↑x1 : Type u) ↪o (↑x2 : Type u)) (CC :=
                      carrier)).map
                b) :
            (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj k)) →
              (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj l))
          y₂ :
        (fun (X : Type u) ↦ X)
          ((F ⋙
                forget PartOrdEmb (FC := fun (x1 x2 : PartOrdEmb.{u}) ↦ (↑x1 : Type u) ↪o (↑x2 : Type u)) (CC :=
                  carrier)).obj
            l))),
  ∃ (j : J) (x' : (↑(F.obj j) : Type u)) (y' : (↑(F.obj j) : Type u)) (_ :
    (ConcreteCategory.hom (FC := TypeCat.Fun) (c.ι.app j) :
          (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj j)) →
            (fun (X : Type u) ↦ X) (((Functor.const J).obj c.pt).obj j))
        x' =
      x)
    (_ :
    (ConcreteCategory.hom (FC := TypeCat.Fun) (c.ι.app j) :
          (a : (fun (X : Type u) ↦ X) ((F ⋙ forget PartOrdEmb).obj j)) →
            (fun (X : Type u) ↦ X) (((Functor.const J).obj c.pt).obj j))
        y' =
      z),
    x' ≤ y'
Tactic: obtain ⟨l, a, b, h⟩ :=
      (Types.FilteredColimit.isColimit_eq_iff _ hc (xi := y₁) (xj := y₂)).1
        (hy₁.trans hy₂.symm)