sclass VStack implements Steppable {
  new L<Computable> stack;
  O latestResult;
  
  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);
    true;
  }
  
  // called from computation to return a value
  void _return(O value default null) {
    latestResult = value;
    removeLast(stack);
  }
  
  // called from computation to tail-call another routine
  void tailCall(Computable computation) {
    removeLast(stack);
    stack.add(computation);
  }
  
  <A> compute(Computable<A> computation) {
    if (computation == null) null;
    push(computation);
    stepAll(this);
    ret latestResult;
  }
}