assertEquals(StringLength.COMPARATOR, keySet.comparator());
    assertEquals(Sets.<@Nullable String>newHashSet(null, "tree"), keySet.headSet("yahoo"));
    assertEquals(newHashSet("google"), keySet.tailSet("yahoo"));
    assertEquals(newHashSet("tree"), keySet.subSet("ask", "yahoo"));
  }

  @GwtIncompatible // SerializableTester
  public void testExplicitComparatorSerialization() {

          "home.netscape.com",
          "pvt.k12.ca.us",
          "state.CA.us",
          "utenti.blah.IT",
          "web.MIT.edu",
          "www.google.com",
          "www.state.pa.us",
          "www4.yahoo.co.uk"
          // keep-sorted end
          );

  private static final ImmutableSet<String> NON_RS =
      ImmutableSet.<String>builder().addAll(NON_PS).addAll(PS_NOT_RS).build();