public void TestGetEnumerator()
        {
            //Assign
            list.AddFirst(1);
            list.AddLast(2);

            //Act
            IEnumerator <Node <int> > i = list.GetEnumerator();

            //Assert
            i.MoveNext();
            Assert.AreEqual(i.Current.Data, 1);
            i.MoveNext();
            Assert.AreEqual(i.Current.Data, 2);
        }