∀ {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 : X ⟶ K.X q) {n : ℤ}
  (h : p + n = q) (n' q' : ℤ) (h' : p + n' = q'),
  CochainComplex.HomComplex.δ n n' (CochainComplex.HomComplex.Cochain.fromSingleMk f h) =
    CochainComplex.HomComplex.Cochain.fromSingleMk (CategoryTheory.CategoryStruct.comp f (K.d q q')) h'