code: @Override
      public Set<String> getAvailableQueryAttributes() {
          final IUsernameAttributeProvider usernameAttributeProvider =
                  super.getUsernameAttributeProvider();
          return Collections.singleton(usernameAttributeProvider.getUsernameAttribute());
      }
docs: Returns an empty <code>Set</code>, per the API documentation, because we don't use any
  attributes in queries.