static Web web_unpackRelationsStartingWith(Web web, S prefix) {
  for (WebRelation rel : values(web.relations)) {
    new L<Lisp> labels;
    for (Lisp l : rel.labels) {
      S s = web.unparseLabel(l);
      if (swic(s, prefix))
        labels.add(web.parseLabel(trim(s.substring(l(prefix)))));
    }
    web_setLabels(rel, labels);
  }
  ret web_dropEmptyRelations(web);
}