∀ (C : Type u_2) (D : Type u_4) [inst : CategoryTheory.Category.{u_1, u_2} C]
  [inst_1 : CategoryTheory.Category.{u_3, u_4} D] (d : D) (X : C),
  CategoryTheory.CostructuredArrow.homMk (CategoryTheory.CategoryStruct.id X) ⋯ =
    CategoryTheory.CategoryStruct.id (CategoryTheory.CostructuredArrow.mk (CategoryTheory.Join.edge X d))