@abc.abstractmethod<EOL><INDENT>def cardinal_to_string(self, c):<DEDENT>
return<EOL>
Gets a string representation of a cardinal.

        :param c: a cardinal value
        :type c: ``cardinal``
        :return: the display string
        :rtype: ``string``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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