static L<Thread> registeredThreads(O o) {
  Map<Thread, Bool> map = cast getOpt(o, "_registerThread_threads");
  if (map == null) ret ll();
  map.size(); // force clean-up
  synchronized(map) { ret asList(keys(map)); }
}

please include function _registerThread.

static L<Thread> registeredThreads() {
  _registerThread_threads.size(); // force clean-up
  ret asList(keys(_registerThread_threads));
}