docs: public static SortedMap<Short, List<MappeableContainer>> groupByKey(
            ImmutableRoaringBitmap... bitmaps) {
      Map<Short, List<MappeableContainer>> grouped = new HashMap<>();
      for (ImmutableRoaringBitmap bitmap : bitmaps) {
        MappeableContainerPointer it = bitmap.highLowContainer.getContainerPointer();
        while (null != it.getContainer()) {
          MappeableContainer container = it.getContainer();
          Short key = it.key();
          List<MappeableContainer> slice = grouped.get(key);
          if (null == slice) {
            slice = new ArrayList<>();
            grouped.put(key, slice);
          }
          slice.add(container);
          it.advance();
        }
      }
      SortedMap<Short, List<MappeableContainer>> sorted = new TreeMap<>(BufferUtil::compareUnsigned);
      sorted.putAll(grouped);
      return sorted;
    }