static <A, B> Map<A, B> cloneMap(Map<A, B> map) {
  if (map == null) ret litmap();
  // assume mutex is equal to collection
  synchronized(map) {
    ret map instanceof TreeMap ? new TreeMap(map, ((TreeMap) map).comparator())
      : map instanceof LinkedHashMap ? new LinkedHashMap(map)
      : new HashMap(map);
  }
}