code: def get_hierarchies(self):
          
          if self.retrieved:
              raise errors.IllegalState('List has already been retrieved.')
          self.retrieved = True
          return objects.HierarchyList(self._results, runtime=self._runtime)
docs: Gets the hierarchy list resulting from the search.
  
          return: (osid.hierarchy.HierarchyList) - the hierarchy list
          raise:  IllegalState - the hierarchy list was already retrieved
          *compliance: mandatory -- This method must be implemented.*