∀ {C : Type u} {inst : CategoryTheory.Category.{v, u} C} [self : CategoryTheory.Preadditive C] (P Q R : C)
  (f f' : P ⟶ Q) (g : Q ⟶ R) {Z : C} (h : R ⟶ Z),
  CategoryTheory.CategoryStruct.comp (f + f') (CategoryTheory.CategoryStruct.comp g h) =
    CategoryTheory.CategoryStruct.comp
      (CategoryTheory.CategoryStruct.comp f g + CategoryTheory.CategoryStruct.comp f' g) h