static void web_clusterClean(Web web) {
  Clusters<S> clusters = clustersForWeb(web);
  if (empty(clusters)) ret;
  for (WebNode node : web_nodesAndRelations(web)) {
    new L<Lisp> labels;
    for (Lisp l : node.labels) {
      S s = first(clusters.get(web.unparseLabel(l)));
      setAdd(labels, s != null ? web.parseLabel(s) : l);
    }
    web_setLabels(node, labels);
  }
}