code: def get_roots(self):
          
          id_list = []
          for r in self._rls.get_relationships_by_genus_type_for_source(self._phantom_root_id, self._relationship_type):
              id_list.append(r.get_destination_id())
          return IdList(id_list)
docs: Gets the root nodes of this hierarchy.
  
          return: (osid.id.IdList) - the root nodes
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*