code: func bitsToRoaringData(ps pairSet) ([]byte, error) {
  	bmp := roaring.NewBitmap()
  	for j := 0; j < len(ps.columnIDs); j++ {
  		bmp.DirectAdd(ps.rowIDs[j]*ShardWidth + (ps.columnIDs[j] % ShardWidth))
  	}
  
  	var buf bytes.Buffer
  	_, err := bmp.WriteTo(&buf)
  	if err != nil {
  		return nil, errors.Wrap(err, "writing to buffer")
  	}
  
  	return buf.Bytes(), nil
  }
docs: // bitsToRoaringData converts a pairSet into a roaring.Bitmap
  // which represents the data within a single shard.