concept PersistentLister implements ILister {
  new HashMap<S, L> lists;
  new HashMap<S, Map> treeMaps;
  
  public synchronized L getList(S name) {
    L l = lists.get(name);
    if (l == null)
      lists.put(name, l = listInConcept(this, new L));
    ret l;
  }
  
  public synchronized Map getTreeMap(S name) {
    Map m = treeMaps.get(name);
    if (m == null)
      treeMaps.put(name, m = mapInConcept(this, new TreeMap));
    ret m;
  }
}