∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Preadditive C]
  (K L : CochainComplex C ℤ) (n : ℤ),
  CochainComplex.HomComplex.coboundaries K L n =
    { carrier := {α | ∃ m, ∃ (_ : m + 1 = n), ∃ β, CochainComplex.HomComplex.δ m n β = ↑α}, add_mem' := ⋯,
      zero_mem' := ⋯, neg_mem' := ⋯ }