docs: public static SortedMap<Short, List<Container>> groupByKey(RoaringBitmap... bitmaps) {
      Map<Short, List<Container>> grouped = new HashMap<>();
      for (RoaringBitmap bitmap : bitmaps) {
        RoaringArray ra = bitmap.highLowContainer;
        for (int i = 0; i < ra.size; ++i) {
          Container container = ra.values[i];
          Short key = ra.keys[i];
          List<Container> slice = grouped.get(key);
          if (null == slice) {
            slice = new ArrayList<>();
            grouped.put(key, slice);
          }
          slice.add(container);
        }
      }
      SortedMap<Short, List<Container>> sorted = new TreeMap<>(Util::compareUnsigned);
      sorted.putAll(grouped);
      return sorted;
    }