∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Preadditive C]
  {F G K L : CochainComplex C ℤ} {n₂ n₃ n₂₃ : ℤ} (z₁ : CochainComplex.HomComplex.Cochain F G 0)
  (z₂ : CochainComplex.HomComplex.Cochain G K n₂) (z₃ : CochainComplex.HomComplex.Cochain K L n₃) (h₂₃ : n₂ + n₃ = n₂₃),
  (z₁.comp z₂ ⋯).comp z₃ h₂₃ = z₁.comp (z₂.comp z₃ h₂₃) ⋯