@abc.abstractmethod<EOL><INDENT>def get_references(self, grade_id):<DEDENT>
return<EOL>
Gets the references with ratings equal to or higher than the given grade.

        :param grade_id: the ``Id`` of the grade
        :type grade_id: ``osid.id.Id``
        :return: the cumulative rating
        :rtype: ``osid.grading.Grade``
        :raise: ``NotFound`` -- no reference found with the given ``Id``
        :raise: ``NullArgument`` -- ``reference_id`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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