docs: def update_question(self, question_form):
          """Updates an existing question.
  
          arg:    question_form (osid.assessment.QuestionForm): the form
                  containing the elements to be updated
          raise:  IllegalState - ``question_form`` already used in an
                  update transaction
          raise:  InvalidArgument - the form contains an invalid value
          raise:  NullArgument - ``question_form`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure occurred
          raise:  Unsupported - ``question_form`` did not originate from
                  ``get_question_form_for_update()``
          *compliance: mandatory -- This method must be implemented.*
  
          """
          collection = JSONClientValidated('assessment',
                                           collection='Item',
                                           runtime=self._runtime)
          if not isinstance(question_form, ABCQuestionForm):
              raise errors.InvalidArgument('argument type is not an QuestionForm')
          if not question_form.is_for_update():
              raise errors.InvalidArgument('the QuestionForm is for update only, not create')
          try:
              if self._forms[question_form.get_id().get_identifier()] == UPDATED:
                  raise errors.IllegalState('question_form already used in an update transaction')
          except KeyError:
              raise errors.Unsupported('question_form did not originate from this session')
          if not question_form.is_valid():
              raise errors.InvalidArgument('one or more of the form elements is invalid')
          item_id = Id(question_form._my_map['itemId']).get_identifier()
          item = collection.find_one({'$and': [{'_id': ObjectId(item_id)},
                                     {'assigned' + self._catalog_name + 'Ids': {'$in': [str(self._catalog_id)]}}]})
          item['question'] = question_form._my_map
          try:
              collection.save(item)
          except:  # what exceptions does mongodb save raise?
              raise errors.OperationFailed()
          self._forms[question_form.get_id().get_identifier()] = UPDATED
          # Note: this is out of spec. The OSIDs don't require an object to be returned:
          return objects.Question(osid_object_map=question_form._my_map,
                                  runtime=self._runtime,
                                  proxy=self._proxy)