{C : Type u} →
  [inst : CategoryTheory.Category.{v, u} C] →
    [inst_1 : CategoryTheory.Abelian C] →
      [inst_2 : CategoryTheory.HasExt C] →
        {X Y Z : C} →
          {a b : ℕ} →
            CategoryTheory.Abelian.Ext X Y a →
              CategoryTheory.Abelian.Ext Y Z b → {c : ℕ} → a + b = c → CategoryTheory.Abelian.Ext X Z c