∀ {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 : C ≌ D} (f : X.functor ⟶ Y.functor),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.id X.functor) f = f