@abc.abstractmethod<EOL><INDENT>def get_cumulative_rating(self):<DEDENT>
return<EOL>
Gets the cumulative rating for all the references in this book.

        :return: the cumulative rating
        :rtype: ``osid.grading.Grade``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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