∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Preadditive C]
  {F G : CochainComplex C ℤ} {n : ℤ} (z₂ : CochainComplex.HomComplex.Cochain F G n),
  (CochainComplex.HomComplex.Cochain.ofHom (CategoryTheory.CategoryStruct.id F)).comp z₂ ⋯ = z₂