docs: def save_to_data(self, in_place=False):
          """Save index to data structure.
  
          :param in_place: Do not copy index value to a new list object
          :type in_place: bool
          :return: Index data structure
          :rtype: list
  
          """
          if in_place:
              return [
                  list(self._index.items()),
                  list(self._undefined_keys.keys())
              ]
          return (
              [(key, values[:]) for key, values in self._index.items()],
              list(self._undefined_keys.keys()),
          )