for (E element : contents) {
        expected.add(element);
      }
    }
    assertEquals(expected, set);
  }

  @GwtIncompatible // NavigableSet
  public void testUnmodifiableNavigableSet() {
    TreeSet<Integer> mod = Sets.newTreeSet();
    mod.add(1);
    mod.add(2);
    mod.add(3);

    NavigableSet<Integer> unmod = unmodifiableNavigableSet(mod);