sclass PStack is Steppable {
  settable new ProbabilisticScheduler scheduler;
  
  sclass NoOptionsException extends RuntimeException {}
  
  srecord noeq ExecuteOption(IVF1 option) is VStack.Computable {
    public void step(VStack stack, O subComputationResult) {
      O target = stack.caller();
      stack.return();
      option.get(target);
    }
  }
  
  class SingleStack extends VStack is IPStack, Runnable {
    double probability = 1.0;
    
    *() {}
    *(PStackComputable computable) { super(computable); }
    
    run {
      if (step())
        // re-add myself to scheduler to continue
        scheduler.add(probability, this);
    }
    
    public <B extends VStack.Computable> void options(B function, Iterable<IVF1<B>> options) {
      L<IVF1<B>> optionsList = nonNulls(options);
      
      if (empty(optionsList))
        throw new NoOptionsException;
        
      // probability penalty according to number of options
      double newProbability = probability/l(optionsList);
        
      // Schedule all options
      for (option : optionsList) {
        new SingleStack s;
        s.stack = shallowCloneElements(stack);
        s.push(new ExecuteOption(option));
        s.probability = newProbability;
        PStack.this.add(s);
      }
    }
  }
  
  srecord noeq FollowUp<A>(PStackComputable<A> computable, IVF1<A> onCompletion) extends PStackComputableWithStep {
    // !customConstructor
    *(PStackComputable<A> *computable, IVF1<A> *onCompletion) {
      probability = computable.probability;
    }
    
    void step(IPStack stack) {
      if (step == 0) {
        stack.push(computable);
        ++step;
      } else {
        onCompletion.get((A) stack.subResult());
        stack.return();
      }
    }
  }
  
  void add(PStackComputable computable) {
    scheduler.at(computable.probability, 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(); }
}