code: def authenticate_multi(self, msg: Dict, signatures: Dict[str, str],
                             threshold: Optional[int] = None):
          
docs: :param msg:
          :param signatures: A mapping from identifiers to signatures.
          :param threshold: The number of successful signature verification
          required. By default all signatures are required to be verified.
          :return: returns the identifiers whose signature was matched and
          correct; a SigningException is raised if threshold was not met