static WeakHashMap<Web, Clusters<S>> clustersForWeb_map = new WeakHashMap;

static Clusters<S> clustersForWeb(Web web) {
  synchronized(clustersForWeb_map) {
    Clusters<S> c = clustersForWeb_map.get(web);
    if (c == null) clustersForWeb_map.put(web, c = new HashMap);
    ret c;
  }
}