Return a json dictionary representing this model.

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