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