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