∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] {X Y : Cᵒᵖ} {f : X ⟶ Y}
  (hf : CategoryTheory.RegularEpi f),
  CategoryTheory.CategoryStruct.comp hf.left f = CategoryTheory.CategoryStruct.comp hf.right f