static <A extends Runnable> void parallelDo(Cl<A> l, O... _) {
  parallelDo(iterator(l), l(l), _);
}

static <A extends Runnable> void parallelDo(Iterator<A> it, int count, O... _) ctex {
  optPar int queueSize = 500;
  optPar int poolSize = coresToUse_fixed();
  NotifyingBlockingThreadPoolExecutor e = new(poolSize, queueSize, 15, TimeUnit.SECONDS);

  // TODO: if poolSize == 1, do it in this thread

  new Var<Throwable> error;
  try {
    for (A o : iterable(it)) {
      if (o == null) continue;
      e.execute(r { pcall {
        try {
          o.run();
        } catch e {
          error.set(e);
        }
      }});
    }
    e.shutdown();
    e.awaitTermination(1, TimeUnit.DAYS);
  } finally {
    e.shutdown();
  }

  if (error.has())
    throw rethrow(error!);
}

Began life as a copy of #1015446