static L<WebRelation> web_findRelations(Web web, S a, S rel, S b) {
  new L<WebRelation> l;
  for (WebRelation r : web_relationObjects(web))
    if (web_matchNodeName(r.a, a) && web_matchNodeName(r.b, b)
      && web_matchNodeName(r, rel))
      l.add(r);
  ret l;
}