static void web_performReplaceX(Web swapWeb, Web web) {
  for (Pair<WebNode> p : web_relationsOfTypeIC(swapWeb, "replace with")) {
    S lbl1 = web_text(web_forwardRelationTo(p.a, "X"));
    S lbl2 = web_text(web_forwardRelationTo(p.b, "X"));
    if (lbl1 != null && lbl2 != null)
      web_replaceLabelsX(web, web_text(p.a), lbl1, web_text(p.b), lbl2);
  }
}