docs: def _to_dict(self):
          """Return a json dictionary representing this model."""
          _dict = {}
          if hasattr(self, 'text') and self.text is not None:
              _dict['text'] = self.text
          if hasattr(self, 'tokens') and self.tokens is not None:
              _dict['tokens'] = self.tokens
          if hasattr(self, 'readings') and self.readings is not None:
              _dict['readings'] = self.readings
          if hasattr(self, 'part_of_speech') and self.part_of_speech is not None:
              _dict['part_of_speech'] = self.part_of_speech
          return _dict