∀ {T : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} T] {X : T} (f : CategoryTheory.Over X)
  (g : CategoryTheory.Over f.left),
  CategoryTheory.CategoryStruct.comp g.hom f.hom =
    (CategoryTheory.Over.mk (CategoryTheory.CategoryStruct.comp g.hom f.hom)).hom