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sclass VStack implements Steppable {
  new L<Computable> stack;
  O latestResult;
  
  // was last instruction a return?
  // can be used by computations to switch without a label
  transient bool justReturned;
  transient O subComputationResult;
  
  // the null sentinel replaces a null function result
  // when stored in latestResult
  sclass NullSentinel {}
  static new NullSentinel nullSentinel;

  *() {}
  *(Computable computation) { push(computation); }
  
  // A is what the function returns
  interface Computable<A> {
    public void step(VStack stack, O subComputationResult);
  }
  
  private O deSentinel(O o) {
    ret o instanceof NullSentinel ? null : o;
  }
  
  private O sentinel(O o) {
    ret o == null ? nullSentinel : o;
  }
  
  // called by computations or users to start a subroutine
  void push(Computable computation) {
    stack.add(computation);
  }
  
  // perform a computation step. returns false iff done
  public bool step() {
    if (empty(stack)) false;
    justReturned = latestResult != null;
    subComputationResult = deSentinel(latestResult);
    latestResult = null;
    last(stack).step(this, subComputationResult);
    subComputationResult = null;
    true;
  }
  
  // called from a computation to return a value
  void _return(O value default null) {
    latestResult = sentinel(value);
    removeLast(stack);
  }
  
  // called from a computation to tail-call another routine
  void tailCall(Computable computation) {
    removeLast(stack);
    stack.add(computation);
  }
 
  // all-in-one evaluation function - call on an empty stack 
  <A> A compute(Computable<A> computation) {
    if (computation == null) null;
    push(computation);
    stepAll(this);
    ret (A) latestResult;
  }
  
  // return result of just completed computation
  O result() {
    ret deSentinel(latestResult);
  }
  
  bool hasSubResult() { ret justReturned; }
}