public boolean apply(Entry<String, String> entry) {
          return entry.getKey() != null && entry.getValue() != null;
        }
      };

  private static class NonNavigableSortedSet extends ForwardingSortedSet<String> {

    private final SortedSet<String> delegate = Sets.newTreeSet(Ordering.natural());

    @Override
    protected SortedSet<String> delegate() {
      return delegate;
    }
  }