∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : HasDerivedCategory C] (T : CategoryTheory.Pretriangulated.Triangle (DerivedCategory C)),
  T ∈ CategoryTheory.Pretriangulated.distinguishedTriangles ↔
    ∃ X Y f, Nonempty (T ≅ DerivedCategory.Q.mapTriangle.obj (CochainComplex.mappingCone.triangle f))