∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.Preadditive C]
  {S₁ S₂ : CategoryTheory.ShortComplex C} {φ₁ φ₂ φ₃ φ₄ : S₁ ⟶ S₂} (h : CategoryTheory.ShortComplex.Homotopy φ₁ φ₂)
  (h' : CategoryTheory.ShortComplex.Homotopy φ₃ φ₄),
  (φ₁ + φ₃).τ₁ = CategoryTheory.CategoryStruct.comp S₁.f (h.h₁ + h'.h₁) + (h.h₀ + h'.h₀) + (φ₂ + φ₄).τ₁