// TODO: it isn't actually multithreaded at all
sclass CalculateEachElementOnce<A, B> {
  Map<A, Flag> calculating = syncMap();
  Map<A, OKOrError<B>> calculated = syncMap();
  
  synchronized B get(A a, IF1<A, B> f) {
    Flag flag = calculating.get(a);
    if (flag != null) {
      flag.waitUntilUp();
      ret unpackOKOrError(calculated.get(a));
    } else {
      flag = new Flag;
      calculating.put(a, flag);
      OKOrError<B> b = okOrError(() -> f.get(a));
      calculated.put(a, b);
      calculating.remove(a);
      flag.raise();
      ret b!;
    }
  }
}