@abc.abstractmethod<EOL><INDENT>def get_authority(self):<DEDENT>
return<EOL>
Gets the authority of this ``Id``.

        The authority is a string used to ensure the uniqueness of this
        ``Id`` when using a non- federated identifier space. Generally,
        it is a service name identifying the provider of this ``Id``.
        This method is used to compare one ``Id`` to another.

        :return: the authority of this ``Id``
        :rtype: ``string``


        *compliance: mandatory -- This method must be implemented.*