∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.HasShift C ℤ]
  {T₁ T₂ : CategoryTheory.Pretriangulated.Triangle C} [inst_2 : CategoryTheory.Preadditive C] (f g : T₁ ⟶ T₂),
  CategoryTheory.CategoryStruct.comp T₁.mor₂ (f.hom₃ + g.hom₃) =
    CategoryTheory.CategoryStruct.comp (f.hom₂ + g.hom₂) T₂.mor₂