static Web web_dropLabelsStartingWith(Web web, S prefix) {
  for (WebNode n : web_nodesAndRelations(web)) {
    for (ListIterator<Lisp> it = n.labels.listIterator(); it.hasNext(); )
      if (swic(web.unparseLabel(it.next()), prefix))
        it.remove();
  }
  ret web;
}