∀ {C : Type u_4} [inst : CategoryTheory.Category.{u_3, u_4} C] {D : Type u_2}
  [inst_1 : CategoryTheory.Category.{u_1, u_2} D] {S : D} {T : CategoryTheory.Functor C D}
  (f : CategoryTheory.StructuredArrow S T),
  CategoryTheory.CategoryStruct.comp f.hom (T.map (CategoryTheory.Iso.refl f.right).hom) =
    (CategoryTheory.StructuredArrow.mk f.hom).hom