static Collection<WebNode> web_singleNodes(Web web) {
  web_dropEmptyRelations(web);
  new HashSet<WebNode> set;
  for (WebRelation r : web_relationObjects(web)) {
    set.add(r.a);
    set.add(r.b);
  }
  ret listMinusSet(web_nodesWithoutRelations(web), set);
}