∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.CartesianMonoidalCategory C]
  (X : C) (Z : CategoryTheory.Over X),
  CategoryTheory.CategoryStruct.comp
      (CategoryTheory.CartesianMonoidalCategory.lift (CategoryTheory.CategoryStruct.id Z.left) Z.hom)
      (((CategoryTheory.Over.forget X).comp (CategoryTheory.toOver X)).obj Z).hom =
    ((CategoryTheory.Functor.id (CategoryTheory.Over X)).obj Z).hom