∀ {C : Type u_4} [inst : CategoryTheory.Category.{u_2, u_4} C] {D : Type u_1}
  [inst_1 : CategoryTheory.Category.{u_3, u_1} D] (e : C ≌ D),
  CategoryTheory.Skeletal D → ∀ (X : D), e.functor.obj (e.inverse.obj X) = X