∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.Preadditive C]
  (K L : CochainComplex C ℤ) (n : ℤ), ∃ m, ∃ (_ : m + 1 = n), ∃ β, CochainComplex.HomComplex.δ m n β = ↑0