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