∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : HasDerivedCategory C],
  ∀ T ∈ CategoryTheory.Pretriangulated.distinguishedTriangles,
    ∀ (n₀ : ℤ),
      CategoryTheory.Epi ((DerivedCategory.homologyFunctor C n₀).map T.mor₁) ↔
        (DerivedCategory.homologyFunctor C n₀).map T.mor₂ = 0