@J2ktIncompatible
  @GwtIncompatible // NullPointerTester
  public void testNullPointers() {
    new NullPointerTester().testAllPublicStaticMethods(ImmutableSortedSet.class);
  }

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

  public void testEmpty_headSet() {
    SortedSet<String> set = of();