Convert this HandwrittenData object into a list of HandwrittenData
        objects. Each element of the list is a single symbol.

        Returns
        -------
        list of HandwrittenData objects

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