∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] (X : C)
  [inst_1 : CategoryTheory.Limits.HasBinaryCoproducts C] (X_1 : (CategoryTheory.coprodMonad X).Algebra),
  CategoryTheory.Under.homMk (CategoryTheory.CategoryStruct.id X_1).f ⋯ =
    CategoryTheory.CategoryStruct.id
      (CategoryTheory.Under.mk (CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.coprod.inl X_1.a))