∀ {C : Type u_1} [inst : CategoryTheory.Category.{v_1, u_1} C] [inst_1 : CategoryTheory.Preadditive C]
  [inst_2 : CategoryTheory.Limits.HasBinaryBiproducts C] {K₁ L₁ K₂ L₂ : CochainComplex C ℤ} {φ₁ : K₁ ⟶ L₁}
  {φ₂ : K₂ ⟶ L₂} {a : K₁ ⟶ K₂} {b : L₁ ⟶ L₂}
  (H : Homotopy (CategoryTheory.CategoryStruct.comp φ₁ b) (CategoryTheory.CategoryStruct.comp a φ₂)),
  (CochainComplex.mappingCone.trianglehMapOfHomotopy H).hom₂ = (HomotopyCategory.quotient C (ComplexShape.up ℤ)).map b