code: def get_grade_system_gradebook_session(self, proxy):
          
          if not self.supports_grade_system_gradebook():
              raise errors.Unimplemented()
          # pylint: disable=no-member
          return sessions.GradeSystemGradebookSession(proxy=proxy, runtime=self._runtime)
docs: Gets the session for retrieving grade system to gradebook mappings.
  
          arg:    proxy (osid.proxy.Proxy): a proxy
          return: (osid.grading.GradeSystemGradebookSession) - a
                  ``GradeSystemGradebookSession``
          raise:  NullArgument - ``proxy`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  Unimplemented - ``supports_grade_system_gradebook()`` is
                  ``false``
          *compliance: optional -- This method must be implemented if
          ``supports_grade_system_gradebook()`` is ``true``.*