∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Preadditive C]
  {F G : CochainComplex C ℤ} (z : CochainComplex.HomComplex.Cochain F G (-1)),
  CochainComplex.HomComplex.δ (-1) 0 z =
    CochainComplex.HomComplex.Cochain.ofHom (Homotopy.nullHomotopicMap' fun i j hij => z.v i j ⋯)