∀ {C : Type u_1} [inst : CategoryTheory.Category.{v_1, u_1} C] [inst_1 : CategoryTheory.Preadditive C]
  [inst_2 : CategoryTheory.Limits.HasBinaryBiproducts C] {K L : CochainComplex C ℤ} (φ : K ⟶ L) (p : ℤ),
  CategoryTheory.CategoryStruct.comp ((CochainComplex.mappingCone.inr φ).f p)
      ((CochainComplex.mappingCone.triangle φ).mor₃.f p) =
    0