SortedSet<String> set = of("e", "a", "f", "b", "d", "c");
    SortedSet<String> headset = set.headSet("d");
    SortedSet<String> copy = copyOf(headset);
    assertEquals(headset, copy);
  }

  public void testCopyOf_tailSet() {
    SortedSet<String> set = of("e", "a", "f", "b", "d", "c");
    SortedSet<String> tailset = set.tailSet("d");
    SortedSet<String> copy = copyOf(tailset);
    assertEquals(tailset, copy);
  }