docs: def delete_grade(self, grade_id):
          """Deletes a ``Grade``.
  
          arg:    grade_id (osid.id.Id): the ``Id`` of the ``Grade`` to
                  remove
          raise:  NotFound - ``grade_id`` not found
          raise:  NullArgument - ``grade_id`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*
  
          """
          # Implemented from template for
          # osid.repository.AssetAdminSession.delete_asset_content_template
          from dlkit.abstract_osid.id.primitives import Id as ABCId
          from .objects import Grade
          collection = JSONClientValidated('grading',
                                           collection='GradeSystem',
                                           runtime=self._runtime)
          if not isinstance(grade_id, ABCId):
              raise errors.InvalidArgument('the argument is not a valid OSID Id')
          grade_system = collection.find_one({'grades._id': ObjectId(grade_id.get_identifier())})
  
          index = 0
          found = False
          for i in grade_system['grades']:
              if i['_id'] == ObjectId(grade_id.get_identifier()):
                  grade_map = grade_system['grades'].pop(index)
              index += 1
              found = True
          if not found:
              raise errors.OperationFailed()
          Grade(
              osid_object_map=grade_map,
              runtime=self._runtime,
              proxy=self._proxy)._delete()
          collection.save(grade_system)