Return a json dictionary representing this model.

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