∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.HasShift C ℤ]
  [inst_2 : CategoryTheory.Preadditive C] (T : CategoryTheory.Pretriangulated.Triangle C) (inr : T.obj₃ ⟶ T.obj₂)
  (fst : T.obj₂ ⟶ T.obj₁),
  CategoryTheory.CategoryStruct.comp fst T.mor₁ + CategoryTheory.CategoryStruct.comp T.mor₂ inr =
      CategoryTheory.CategoryStruct.id T.obj₂ →
    CategoryTheory.CategoryStruct.comp fst T.mor₁ =
      CategoryTheory.CategoryStruct.id T.obj₂ - CategoryTheory.CategoryStruct.comp T.mor₂ inr