docs: def to_single_symbol_list(self):
          """
          Convert this HandwrittenData object into a list of HandwrittenData
          objects. Each element of the list is a single symbol.
  
          Returns
          -------
          list of HandwrittenData objects
          """
          symbol_stream = getattr(self,
                                  'symbol_stream',
                                  [None for symbol in self.segmentation])
          single_symbols = []
          pointlist = self.get_sorted_pointlist()
          for stroke_indices, label in zip(self.segmentation, symbol_stream):
              strokes = []
              for stroke_index in stroke_indices:
                  strokes.append(pointlist[stroke_index])
              single_symbols.append(HandwrittenData(json.dumps(strokes),
                                                    formula_id=label))
          return single_symbols