public void testEmpty_serialization() {
    SortedSet<String> set = of();
    SortedSet<String> copy = SerializableTester.reserialize(set);
    assertSame(set, copy);
  }

  public void testSingle_comparator() {
    SortedSet<String> set = of("e");
    assertSame(Ordering.natural(), set.comparator());
  }

  public void testSingle_headSet() {
    SortedSet<String> set = of("e");