∀ {C : Type u_3} [inst : CategoryTheory.Category.{u_1, u_3} C] {D : Type u_4}
  [inst_1 : CategoryTheory.Category.{u_2, u_4} D] {X Y Z : CategoryTheory.Functor C D} (f : X ⟶ Y) (g : Y ⟶ Z),
  CategoryTheory.NatTrans.mapSquare (CategoryTheory.CategoryStruct.comp f g) =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.NatTrans.mapSquare f) (CategoryTheory.NatTrans.mapSquare g)