∀ {X Y Z : CategoryTheory.Grpd} (f : X ⟶ Y) (g : Y ⟶ Z),
  CategoryTheory.Functor.toCatHom (CategoryTheory.CategoryStruct.comp f g) =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.Functor.toCatHom f) (CategoryTheory.Functor.toCatHom g)