∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.Preadditive C]
  (M N K : CategoryTheory.Mat_ C) (f : M ⟶ N) (g g' : N ⟶ K),
  CategoryTheory.CategoryStruct.comp f (g + g') =
    CategoryTheory.CategoryStruct.comp f g + CategoryTheory.CategoryStruct.comp f g'