}

  public void testOf_comparator() {
    SortedSet<String> set = of("e", "a", "f", "b", "d", "c");
    assertSame(Ordering.natural(), set.comparator());
  }

  public void testOf_headSet() {
    SortedSet<String> set = of("e", "f", "b", "d", "c");
    assertTrue(set.headSet("e") instanceof ImmutableSortedSet);
    assertThat(set.headSet("e")).containsExactly("b", "c", "d").inOrder();