code: def restore(self, res_id, backup_snap=None):
          
          name = self._get_name()
          out = self._cli.restore_snap(name, res_id, backup_snap)
          ex.raise_if_err(out, 'failed to restore snap {}.'.format(name),
                          default=ex.VNXSnapError)
docs: Restores a snapshot.
          :param res_id: the LUN number of primary LUN or snapshot mount point to
              be restored.
          :param backup_snap: the name of a backup snapshot to be created before
              restoring.