static L<Thread> allThreads_fast() {
  ThreadGroup rootGroup = rootThreadGroup();
  Thread[] threads = new Thread[rootGroup.activeCount()+10];
  int n;
  while ((n = rootGroup.enumerate(threads, true)) == threads.length)
    threads = new Thread[threads.length*2];
  L<Thread> l = new ArrayList<Thread>(n);
  for (int i = 0; i < n; i++) l.add(threads[i]);
  ret l;
}