∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.Preadditive C] {X Y : C}
  {f g : X ⟶ Y} (c : CategoryTheory.Limits.Fork f g),
  CategoryTheory.CategoryStruct.comp c.ι (f - g) = CategoryTheory.CategoryStruct.comp c.ι 0