def METHOD_NAME(self, asn: int) -> AutonomousSystem:
    """!
    @brief Create a new AutonomousSystem.
    @param asn ASN of the new AS.
    @returns created AS.
    @throws AssertionError if asn exists.
    """
    assert asn not in self.__ases, "as{} already exist.".format(asn)
    self.__ases[asn] = AutonomousSystem(asn)
    return self.__ases[asn]