}

  public void testEmpty_headSet() {
    SortedSet<String> set = of();
    assertThat(set.headSet("c")).isSameInstanceAs(set);
  }

  public void testEmpty_tailSet() {
    SortedSet<String> set = of();
    assertThat(set.tailSet("f")).isSameInstanceAs(set);
  }

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