docs: def MultiMarkForDeletion(self, urns):
      """Marks multiple urns (and their children) for deletion."""
      all_children_urns = self.RecursiveMultiListChildren(urns)
  
      urns.extend(collection.Flatten(itervalues(all_children_urns)))
      self._urns_for_deletion.update(urns)
  
      for obj in self.MultiOpen(urns):
        obj.OnDelete(deletion_pool=self)