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