∀ {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 : S₁ ⟶ S₂} (h₁₂ : CategoryTheory.ShortComplex.Homotopy φ₁ φ₂)
  (h₂₃ : CategoryTheory.ShortComplex.Homotopy φ₂ φ₃), (h₁₂.trans h₂₃).h₁ = h₁₂.h₁ + h₂₃.h₁