public int hashCode() {
      // Warning: this is broken if size() == 0, so it is critical that we
      // substitute an empty ImmutableSet to the user in place of this

      // It's a weird formula, but tests prove it works.
      int adjust = size() - 1;
      for (int i = 0; i < axes.size(); i++) {
        adjust *= 31;
        adjust = ~~adjust;
        // in GWT, we have to deal with integer overflow carefully

      return topMetaIterator;
    }

    @Override
    public boolean hasNext() {
      while (!checkNotNull(iterator).hasNext()) {
        // this weird checkNotNull positioning appears required by our tests, which expect
        // both hasNext and next to throw NPE if an input iterator is null.

        topMetaIterator = getTopMetaIterator();
        if (topMetaIterator == null) {
          return false;