static void web_replaceLabelsX(Web web, S a, S lbl1, S b, S lbl2) {
  for (Pair<WebNode> p : keys(web.relations))
    if (web_hasLabelIC(p.a, a) && web_hasLabelIC(web.relations.get(p), lbl1)) {
      p.a.labels = ll(web.parseLabel(b));
      web.relations.get(p).labels = ll(web.parseLabel(lbl2));
    }
}