∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.Preadditive C]
  (S : CategoryTheory.ShortComplex C),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.id S) (CategoryTheory.CategoryStruct.id S) =
    CategoryTheory.CategoryStruct.id S