public void RemoveTest()
        {
            ImmutableTrieList <int> list = ImmutableTrieList <int> .Empty;

            for (int i = 1; i <= 10; i++)
            {
                list = list.Add(i * 10);
            }

            list = list.Remove(30);
            Assert.Equal(9, list.Count);
            Assert.False(list.Contains(30));

            list = list.Remove(100);
            Assert.Equal(8, list.Count);
            Assert.False(list.Contains(100));

            list = list.Remove(10);
            Assert.Equal(7, list.Count);
            Assert.False(list.Contains(10));

            var removeList = new int[] { 20, 70 };

            list = list.RemoveAll(item => removeList.Contains(item));
            Assert.Equal(5, list.Count);
            Assert.False(list.Contains(20));
            Assert.False(list.Contains(70));

            IImmutableList <int> list2 = ImmutableTrieList <int> .Empty;

            for (int i = 1; i <= 10; i++)
            {
                list2 = list2.Add(i * 10);
            }

            list2 = list2.Remove(30);
            Assert.Equal(9, list2.Count);
            Assert.False(list2.Contains(30));

            list2 = list2.Remove(100);
            Assert.Equal(8, list2.Count);
            Assert.False(list2.Contains(100));

            list2 = list2.Remove(10);
            Assert.Equal(7, list2.Count);
            Assert.False(list2.Contains(10));

            list2 = list2.RemoveAll(item => removeList.Contains(item));
            Assert.Equal(5, list2.Count);
            Assert.False(list2.Contains(20));
            Assert.False(list2.Contains(70));
        }