∀ {C : Type u_1} [inst : CategoryTheory.Category.{v_1, u_1} 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), (h.add h').h₁ = h.h₁ + h'.h₁