∀ {C : Type u_1} [inst : CategoryTheory.Category.{v, u_1} C] [inst_1 : CategoryTheory.Preadditive C]
  [inst_2 : CategoryTheory.Limits.HasBinaryBiproducts C] {X₁ X₂ X₃ : CochainComplex C ℤ} (f : X₁ ⟶ X₂) (g : X₂ ⟶ X₃),
  (CochainComplex.mappingConeCompTriangle f g).mor₂ =
    CochainComplex.mappingCone.map (CategoryTheory.CategoryStruct.comp f g) g f (CategoryTheory.CategoryStruct.id X₃) ⋯