// filters in parallel, returns elements in correct order
static <A> L<A> parallelFilter(Iterable<A> it, IPred<A> f) ctex {
  if (it == null) null;
  new L<Pair<A, Int>> out;
    
  int poolSize = coresToUse_fixed(), queueSize = 500;
  if (poolSize <= 1) ret filter(it, f);
  NotifyingBlockingThreadPoolExecutor e = new(poolSize, queueSize, 15, TimeUnit.SECONDS);

  try {
    int i = 0;
    for (A o : it) {
      ++i;
      int _i = i;
      e.execute(r { pcall {
        if (f.get(o)) synchronized(out) {
          out.add(pair(o, _i));
        }
      }});
    }
    e.shutdown();
    e.awaitTermination(1, TimeUnit.DAYS);
  } finally {
    e.shutdown();
  }

  ret firstOfPairs(sortBySecondOfPairs_inPlace(out));
}