∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Preadditive C]
  {K L : CochainComplex C ℤ} {p q : ℤ} (f : K.X p ⟶ L.X q) (n : ℤ) (hpq : p + n = q),
  (CochainComplex.HomComplex.Cochain.single f n).v p q hpq = f