static L<Pair<WebNode>> webs_matchNodes(Web a, Web b) {
  Map<S, WebNode> index1 = web_textIndex(a);
  Map<S, WebNode> index2 = web_textIndex(b);
  new L<Pair<WebNode>> out;
  for (S text : setIntersection(keys(index1), keys(index2)))
    out.add(pair(index1.get(text), index2.get(text)));
  ret out;
}