{C : Type u} →
  [inst : CategoryTheory.Category.{v, u} C] →
    [inst_1 : CategoryTheory.Abelian C] →
      [inst_2 : CategoryTheory.HasExt C] →
        {X Y : C} →
          (R : CategoryTheory.ProjectiveResolution X) →
            {n : ℕ} →
              (f : R.complex.X n ⟶ Y) →
                (m : ℕ) →
                  n + 1 = m →
                    CategoryTheory.CategoryStruct.comp (R.complex.d m n) f = 0 → CategoryTheory.Abelian.Ext X Y n