static <A extends Collection<? extends Collection>> L parallelMap(A l, fO f) ctex {
  ret parallelMap(iterator((Collection) l), f);
}

static L parallelMap(Iterator<? extends Collection> it, fO f) ctex {
  final new L<Pair<O, Int>> out;
    
  int poolSize = coresToUse(), queueSize = 10;
  NotifyingBlockingThreadPoolExecutor e = new(poolSize, queueSize, 15, TimeUnit.SECONDS);
    
  try {
    int i = 0;
    for (fO o : iterable(it)) {
      ++i;
      final int _i = i;
      e.execute(r { pcall {
        O x = callF(f, o);
        synchronized(out) {
          out.add(pair(x, _i));
        }
      }});
    }
    e.shutdown();
    e.awaitTermination(1, TimeUnit.DAYS);
  } finally {
    e.shutdown();
  }
    
  ret firstOfPairs(sortBySecondOfPairs_inPlace(out));
}