Return a json dictionary representing this model.

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