∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Preadditive C]
  {F G K : CochainComplex C ℤ} {n : ℤ} (z₁ : CochainComplex.HomComplex.Cochain F G n) (f : G ⟶ K) (m : ℤ),
  CochainComplex.HomComplex.δ n m (z₁.comp (CochainComplex.HomComplex.Cochain.ofHom f) ⋯) =
    (CochainComplex.HomComplex.δ n m z₁).comp (CochainComplex.HomComplex.Cochain.ofHom f) ⋯