docs: public static MutableRoaringBitmap priorityqueue_xor(ImmutableRoaringBitmap... bitmaps) {
      // code could be faster, see priorityqueue_or
      if (bitmaps.length < 2) {
        throw new IllegalArgumentException("Expecting at least 2 bitmaps");
      }
      final PriorityQueue<ImmutableRoaringBitmap> pq =
          new PriorityQueue<>(bitmaps.length, new Comparator<ImmutableRoaringBitmap>() {
            @Override
            public int compare(ImmutableRoaringBitmap a, ImmutableRoaringBitmap b) {
              return (int)(a.getLongSizeInBytes() - b.getLongSizeInBytes());
            }
          });
      Collections.addAll(pq, bitmaps);
      while (pq.size() > 1) {
        final ImmutableRoaringBitmap x1 = pq.poll();
        final ImmutableRoaringBitmap x2 = pq.poll();
        pq.add(ImmutableRoaringBitmap.xor(x1, x2));
      }
      return (MutableRoaringBitmap) pq.poll();
    }