static <A, B> A keyForValue_sync(B value, Map<A, B> map) {
  ret keyForValue_sync(map, value);
}

static <A, B> A keyForValue_sync(Map<A, B> map, B value) {
  if (map == null || value == null) null;
  synchronized(map) {
    ret keyForValue(map, value);
  }
}