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