static Set<Pair<WebNode>> web_relations(Web web) {
  if (web.relations == null) {
    new L<Pair<WebNode>> l;
    for (WebNode n : web.nodes)
      if (n instanceof WebRelation)
        l.add(pair(n/WebRelation.a, n/WebRelation.b));
    ret l;
  }
  ret keys(web.relations);
}