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