code: func (f *fragment) pos(rowID, columnID uint64) (uint64, error) {
  	// Return an error if the column ID is out of the range of the fragment's shard.
  	minColumnID := f.shard * ShardWidth
  	if columnID < minColumnID || columnID >= minColumnID+ShardWidth {
  		return 0, errors.Errorf("column:%d out of bounds", columnID)
  	}
  	return pos(rowID, columnID), nil
  }
docs: // pos translates the row ID and column ID into a position in the storage bitmap.