∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] {X Y X' Y' : C} {f : X ⟶ Y} {g : X' ⟶ Y'}
  (e : CategoryTheory.Arrow.mk f ≅ CategoryTheory.Arrow.mk g) (h : CategoryTheory.RegularEpi f),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.Arrow.leftFunc.mapIso e).inv
      (CategoryTheory.CategoryStruct.comp (CategoryTheory.Limits.Cofork.ofπ f ⋯).π
        (CategoryTheory.Arrow.rightFunc.mapIso e).hom) =
    (CategoryTheory.Limits.Cofork.ofπ g ⋯).π