docs: def _to_dict(self):
          """Return a json dictionary representing this model."""
          _dict = {}
          if hasattr(self, 'response_type') and self.response_type is not None:
              _dict['response_type'] = self.response_type
          if hasattr(self, 'values') and self.values is not None:
              _dict['values'] = [x._to_dict() for x in self.values]
          if hasattr(self,
                     'selection_policy') and self.selection_policy is not None:
              _dict['selection_policy'] = self.selection_policy
          if hasattr(self, 'delimiter') and self.delimiter is not None:
              _dict['delimiter'] = self.delimiter
          if hasattr(self, 'time') and self.time is not None:
              _dict['time'] = self.time
          if hasattr(self, 'typing') and self.typing is not None:
              _dict['typing'] = self.typing
          if hasattr(self, 'source') and self.source is not None:
              _dict['source'] = self.source
          if hasattr(self, 'title') and self.title is not None:
              _dict['title'] = self.title
          if hasattr(self, 'description') and self.description is not None:
              _dict['description'] = self.description
          if hasattr(self, 'preference') and self.preference is not None:
              _dict['preference'] = self.preference
          if hasattr(self, 'options') and self.options is not None:
              _dict['options'] = [x._to_dict() for x in self.options]
          if hasattr(self, 'message_to_human_agent'
                    ) and self.message_to_human_agent is not None:
              _dict['message_to_human_agent'] = self.message_to_human_agent
          return _dict