Return a json dictionary representing this model.

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