-- Lean 4 proof step from Mathlib.Topology.Category.Stonean.Basic (goal)
Goal:
X B C : Stonean
φ : X ⟶ C
f : B ⟶ C
inst : Epi f
this :
  ExtremallyDisconnected.{u_1}
    (↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1)
hf :
  Function.Surjective (α :=
    (fun (X : CompHausLike fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1)) ↦
        (↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1))
      B)
    (β :=
    (fun (X : CompHausLike fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1)) ↦
        (↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1))
      C)
    (⇑(ConcreteCategory.hom (FC :=
          fun (x1 x2 : CompHausLike fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1)) ↦
          C((↑(x1.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1),
            (↑(x2.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1)))
          (CC := fun (X : CompHausLike fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1)) ↦
          (↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1)) f) :
      (a :
          (fun (X : CompHausLike fun (X : TopCat) ↦ ExtremallyDisconnected (↑X : Type u_1)) ↦ (↑X.toTop : Type u_1))
            B) →
        (fun (X : CompHausLike fun (X : TopCat) ↦ ExtremallyDisconnected (↑X : Type u_1)) ↦ (↑X.toTop : Type u_1)) C)
f' :
  (↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1) →
    (↑(B.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1)
h :
  Continuous f' ∧
    Eq (α :=
      (↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1) →
        (↑(C.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1))
      ((⇑(TopCat.Hom.hom f.hom) : (a : (↑B.toTop : Type u_1)) → (↑C.toTop : Type u_1)) ∘ f')
      (⇑(TopCat.Hom.hom φ.hom) : (a : (↑X.toTop : Type u_1)) → (↑C.toTop : Type u_1))
⊢ ofHom (fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1)) { toFun := f', continuous_toFun := ⋯ } ≫
      f =
    φ
have h₁ : ∀ (a : (↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1)),
  Eq (α := (↑(C.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1))
    (((ConcreteCategory.hom (FC :=
              fun (x1 x2 : CompHausLike fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1)) ↦
              C((↑(x1.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1),
                (↑(x2.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1)))
              (CC := fun (X : CompHausLike fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1)) ↦
              (↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1))
              (X := X)
              (ofHom (fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))
                    { toFun := f', continuous_toFun := sorry } ≫
                  f :
                X ⟶ C) :
            C((↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1),
              (↑(C.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1))) :
          (a : (↑X.toTop : Type u_1)) → (↑C.toTop : Type u_1))
        a :
      (↑(C.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1))
    ((ConcreteCategory.hom (FC :=
            fun (x1 x2 : CompHausLike fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1)) ↦
            C((↑(x1.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1),
              (↑(x2.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1)))
            (CC := fun (X : CompHausLike fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1)) ↦
            (↑(X.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1)) φ :
          (a : (↑X.toTop : Type u_1)) → (↑C.toTop : Type u_1))
        a :
      (↑(C.toTop (P := fun (X : TopCat.{u_1}) ↦ ExtremallyDisconnected.{u_1} (↑X : Type u_1))) : Type u_1))
Tactic: ext