sclass SerializeComputations<A, B> {
  Map<A, Var<OKOrError<B>>> calculating = syncMap();
  new MultiSetMap<A, Thread> waitingFor;
  
  B get(A a, IF1<A, B> f) {
    ret get(a, () -> f.get(a));
  }

  B get(A a, IF0<B> f) {
    bool meCalc;
    Var<OKOrError<B>> var;
    synchronized {
      var = calculating.get(a);
      meCalc = var == null;
      
      if (meCalc) {
        var = new Var;
        calculating.put(a, var);
      } else
        waitingFor.put(a, currentThread());
    }
    
    if (meCalc) {
      OKOrError<B> b = okOrError(() -> f!);
      var.set(b);
      ret b.getMandatory();
    } else {
      try {
        waitUntilNotNull(var);
        ret var->getMandatory();
      } finally {
        synchronized {
          waitingFor.remove(a, currentThread());
          if (!waitingFor.containsKey(a))
            calculating.remove(a);
        }
      }
    }
  }
}