∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {X Y Z W : C} {f : X ⟶ Y} {g : Z ⟶ W}
  (h : CategoryTheory.RetractArrow f g) {Z_1 : C} (h_1 : W ⟶ Z_1),
  CategoryTheory.CategoryStruct.comp h.i.left (CategoryTheory.CategoryStruct.comp g h_1) =
    CategoryTheory.CategoryStruct.comp f (CategoryTheory.CategoryStruct.comp h.i.right h_1)