public void TestRemoveWhere()
{
var set = new TreeSet <int>(4, Enumerable.Range(0, 10), comparer: null);
Assert.Throws <ArgumentNullException>(() => set.RemoveWhere(null));
Assert.Equal(5, set.RemoveWhere(i => (i % 2) == 0));
Assert.Equal(new[] { 1, 3, 5, 7, 9 }, set);
Assert.Equal(0, set.RemoveWhere(i => i < 0));
Assert.Equal(5, set.Count);
}