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); }

  *(Iterable<Computable> l) { pushAll(l); }

  // 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 aka call(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 aka replace(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 aka subResult() {

    ret deSentinel(latestResult);

  }

  bool hasSubResult() { ret justReturned; }

  void pushAll(Iterable<Computable> l) {

    fOr (Computable c : l)

      push(c);

  }

}