sclass VStack implements Steppable {

  new L<Computable> stack;

  bool justReturned;

  O latestResult;

  transient bool justReturned_new;

  *() {}

  *(Computable computation) { push(computation); }

  // A is what the function returns

  interface Computable<A> {

    public void step(VStack stack, O subComputationResult);

  }

  void push(Computable computation) {

    stack.add(computation);

  }

  // perform a computation step. returns false iff done

  public bool step() {

    if (empty(stack)) false;

    O latestResult = this.latestResult;

    this.latestResult = null;

    last(stack).step(this, latestResult);

    justReturned = justReturned_new;

    justReturned_new = false;

    true;

  }

  // called from computation to return a value

  void _return(O value default null) {

    latestResult = value;

    set justReturned_new;

    removeLast(stack);

  }

  // called from computation to tail-call another routine

  void tailCall(Computable computation) {

    removeLast(stack);

    stack.add(computation);

  }

  <A> A compute(Computable<A> computation) {

    if (computation == null) null;

    push(computation);

    stepAll(this);

    ret (A) latestResult;

  }

  O result() {

    ret latestResult;

  }

}