sclass PStack is Steppable {
  new ProbabilisticScheduler scheduler;
  
  class SingleStack extends VStack is Runnable {
    double probability = 1.0;
    
    *(PStackComputable computable) {
      super(computable);
    }
    
    run {
      if (step())
        // re-add myself to scheduler to continue
        scheduler.add(probability, this);
    }
  }
  
  srecord neoq FollowUp(PStackComputable<A> computable, IVF1<A> onCompletion) extends PStackComputableWithStep {
    void step(PStack stack) {
      if (step == 0) {
        stack.push(computable);
        ++step;
      } else {
        onCompletion.get(stack.subResult());
        stack.return();
      }
    }
  }
  
  void add(PStackComputable computable) {
    scheduler.at(1.0, new SingleStack(computable));
  }
  
  <A> void add(PStackComputable<A> computable, IVF1<A> onCompletion) {
    if (onCompletion != null)
      add(new FollowUp<>(computable, onCompletion));
    else
      add(computable);
  }
  
  public bool step() { ret scheduler.step(); }
}