docs: def _to_dict(self):
          """Return a json dictionary representing this model."""
          _dict = {}
          if hasattr(self, 'utterance_id') and self.utterance_id is not None:
              _dict['utterance_id'] = self.utterance_id
          if hasattr(self, 'utterance_text') and self.utterance_text is not None:
              _dict['utterance_text'] = self.utterance_text
          if hasattr(self, 'tones') and self.tones is not None:
              _dict['tones'] = [x._to_dict() for x in self.tones]
          if hasattr(self, 'error') and self.error is not None:
              _dict['error'] = self.error
          return _dict