public void TestReverse()
        {
            var random = new Random();

            ImmutableSortedTreeSet <int> .Builder set = ImmutableSortedTreeSet.CreateBuilder <int>();
            for (int i = 0; i < 4 * 8 * 8; i++)
            {
                int item = random.Next();
                set.Add(item);
            }

            IEnumerable <int> reversed = set.Reverse();

            Assert.Equal(set.AsEnumerable().Reverse(), reversed);
            Assert.Equal(set, reversed.Reverse());
        }