static Map<S, UniqueString> uniqueString_map = weakHashMap();

static UniqueString uniqueString(S s) {
  if (s == null) null;
  synchronized(uniqueString_map) {
    UniqueString us = uniqueString_map.get(s);
    if (us == null)
      uniqueString_map.put(s, us = UniqueString(s, true));
    ret us;
  }
}

static UniqueString uniqueString(CharSequence s) {
  ret s == null ? null : uniqueString(str(s));
}