CollectionFeature.ALLOWS_NULL_QUERIES)
            .createTestSuite());

    suite.addTestSuite(ImmutableSortedSetTest.class);

    return suite;
  }

  // enum singleton pattern
  private enum StringLengthComparator implements Comparator<String> {
    INSTANCE;

    @Override
    public int compare(String a, String b) {
      return a.length() - b.length();
    }
  }