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

        :param duration: a duration value
        :type duration: ``osid.calendaring.Duration``
        :return: the display string
        :rtype: ``string``
        :raise: ``InvalidArgument`` -- ``duration.get_calendar_type() !=``  ``get_calendar_type()`` or ``duration.get_time_type() != get_time_type()``
        :raise: ``NullArgument`` -- ``duration`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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