SortedSet<String> tailset = set.tailSet("d");
    SortedSet<String> copy = copyOf(tailset);
    assertEquals(tailset, copy);
  }

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

  public void testCopyOf_iterator_ordering() {