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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_grade_systems(self):
 """Gets all ``GradeSystems``. In plenary mode, the returned list contains all known grade systems or an error results. Otherwise, the returned list may contain only those grade systems that are accessible through this session. return: (osid.grading.GradeSystemList) - a ``GradeSystemList`` raise: OperationFailed - unable to complete request raise: PermissionDenied - authorization failure *compliance: mandatory -- This method must be implemented.* """

        # Implemented from template for
        # osid.resource.ResourceLookupSession.get_resources
        # NOTE: This implementation currently ignores plenary view
        collection = JSONClientValidated('grading',
                                         collection='GradeSystem',
                                         runtime=self._runtime)
        result = collection.find(self._view_filter()).sort('_id', DESCENDING)
        return objects.GradeSystemList(result, runtime=self._runtime, proxy=self._proxy)