def get_code(self):
        """
        the final code for multi-checks

        arguments:
            the worst-case code from all added results,
            or UNKNOWN if none were added
        """
        code = UNKNOWN
        for result in self._results:
            if code == UNKNOWN or (result.code < UNKNOWN
                                   and result.code > code):
                code = result.code
        return code