sclass PStack is Steppable {
  new ProbabilisticScheduler scheduler;
  
  class SingleStack extends VStack is Runnable {
    double probability;
    
    *(double *probability, PStackComputable computable) {
      super(computable);
    }
    
    run {
      if (step())
        // re-add myself to scheduler to continue
        scheduler.add(probability, this);
    }
  }
  
  void addComputation(double probability default 1.0, PStackComputable computable) {
    scheduler.add(probability, new SingleStack(probability, computable));
  }
  
  public bool step() { ret scheduler.step(); }
}