<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def order_items(self, item_ids, assessment_id):
 """Sequences existing items in an assessment. arg: item_ids (osid.id.Id[]):
 the ``Id`` of the ``Items`` arg: assessment_id (osid.id.Id):
 the ``Id`` of the ``Assessment`` raise: NotFound - ``assessment_id`` is not found or an ``item_id`` is not on ``assessment_id`` raise: NullArgument - ``assessment_id`` or ``item_ids`` is ``null`` raise: OperationFailed - unable to complete request raise: PermissionDenied - authorization failure occurred *compliance: mandatory -- This method must be implemented.* """

        if assessment_id.get_identifier_namespace() != 'assessment.Assessment':
            raise errors.InvalidArgument
        self._part_item_design_session.order_items(item_ids, self._get_first_part_id(assessment_id))