code: def get_grade_ids(self):
          
          id_list = []
          for grade_map in self._my_map['grades']:
              id_list.append(Id(grade_map.id))
          if id_list == []:
              raise IllegalState()
          else:
              return IdList(id_list)
docs: Gets the grade Ids in this system ranked from highest to lowest.
  
          return: (osid.id.IdList) - the list of grades Ids
          raise:  IllegalState - is_based_on_grades() is false
          compliance: mandatory - This method must be implemented.