docs: def is_child_of_objective(self, id_=None, objective_id=None):
          """Tests if an objective is a direct child of another.
  
          arg:    id (osid.id.Id): an Id
          arg:    objective_id (osid.id.Id): the Id of an objective
          return: (boolean) - true if the id is a child of objective_id,
                  false otherwise
          raise:  NotFound - objective_id is not found
          raise:  NullArgument - id or objective_id is null
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          compliance: mandatory - This method must be implemented.
          implementation notes: If id not found return false.
  
          """
          if id_ is None or objective_id is None:
              raise NullArgument()
          return id_ in list(self.get_child_objective_ids(objective_id))