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