@abc.abstractmethod<EOL><INDENT>def set_level(self, grade_id):<DEDENT>
pass<EOL>
Sets the level of difficulty expressed as a ``Grade``.

        :param grade_id: the grade level
        :type grade_id: ``osid.id.Id``
        :raise: ``InvalidArgument`` -- ``grade_id`` is invalid
        :raise: ``NoAccess`` -- ``Metadata.isReadOnly()`` is ``true``

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