∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} 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) {X' : C} (g : X ⟶ X'),
  CochainComplex.HomComplex.Cochain.toSingleMk (CategoryTheory.CategoryStruct.comp f g) h =
    (CochainComplex.HomComplex.Cochain.toSingleMk f h).comp
      (CochainComplex.HomComplex.Cochain.ofHom ((CochainComplex.singleFunctor C q).map g)) ⋯