docs: def _to_dict(self):
          """Return a json dictionary representing this model."""
          _dict = {}
          if hasattr(self, 'model_id') and self.model_id is not None:
              _dict['model_id'] = self.model_id
          if hasattr(self, 'model_version') and self.model_version is not None:
              _dict['model_version'] = self.model_version
          if hasattr(self, 'documents') and self.documents is not None:
              _dict['documents'] = [x._to_dict() for x in self.documents]
          if hasattr(self,
                     'aligned_elements') and self.aligned_elements is not None:
              _dict['aligned_elements'] = [
                  x._to_dict() for x in self.aligned_elements
              ]
          if hasattr(
                  self,
                  'unaligned_elements') and self.unaligned_elements is not None:
              _dict['unaligned_elements'] = [
                  x._to_dict() for x in self.unaligned_elements
              ]
          return _dict