// Either calculation result or Thread with timed-out computation
// Tries to interrupt thread after timeout
static Either<O, Thread> evalWithTimeout(int timeoutMS, fO r) {
  final new Flag done;
  final new Var var;
  final new Var<Throwable> error;
  Thread t = startThread(r {
    try {
      var.set(callF(r));
    } catch e {
      error.set(e);
    }
    done.raise();
  });
  done.waitUntilUp(timeoutMS);
  if (!done.isUp()) {
    t.interrupt();
    ret either2(t);
  }
  if (error! != null) rethrow(error!);
  ret either1(var!);
}