∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.Preadditive C]
  [inst_2 : CategoryTheory.Limits.HasZeroObject C] {X : C} {K : CochainComplex C ℤ} {p q : ℤ} (f : K.X p ⟶ X) {n : ℤ}
  (h : p + n = q) (p' : ℤ),
  p' + 1 = p →
    CategoryTheory.CategoryStruct.comp (K.d p' p) f = 0 →
      CochainComplex.HomComplex.δ n (n + 1) (CochainComplex.HomComplex.Cochain.toSingleMk f h) = 0