∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Preadditive C]
  {F G K : CochainComplex C ℤ} {n₁ : ℤ} (z₁ : CochainComplex.HomComplex.Cochain F G n₁)
  (z₂ : CochainComplex.HomComplex.Cochain G K 0) (m₁ : ℤ) (h₁ : n₁ + 1 = m₁),
  CochainComplex.HomComplex.δ n₁ m₁ (z₁.comp z₂ ⋯) =
    z₁.comp (CochainComplex.HomComplex.δ 0 1 z₂) h₁ + (CochainComplex.HomComplex.δ n₁ m₁ z₁).comp z₂ ⋯