assertSame(set, set.tailSet("f"));
  }

  public void testEmpty_subSet() {
    SortedSet<String> set = of();
    assertSame(set, set.subSet("c", "f"));
  }

  public void testEmpty_first() {
    SortedSet<String> set = of();
    assertThrows(NoSuchElementException.class, () -> set.first());
  }

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