public void TestClear()
        {
            ImmutableSortedTreeSet <int> .Builder set = ImmutableSortedTreeSet.CreateBuilder <int>();

            set.Clear();
            Assert.Empty(set);

            set.UnionWith(Enumerable.Range(0, 10));
            Assert.NotEmpty(set);
            set.Clear();
            Assert.Empty(set);
        }