static L<Thread> threadInfosToThreads(L<ThreadInfo> l) {
  new HashMap<Long, Thread> map;
  for (Thread t : keys(Thread.getAllStackTraces()))
    map.put(t.getId(), t);
  new L<Thread> out;
  for (ThreadInfo t : l)
    addIfNotNull(out, map.get(t.getThreadId()));
  ret out;
}