interface IStackWithOptions extends IVStack {
  // declare a choice point for backtracking
  // (or a probabilistic choice point if it's a PStack)
  default <B extends VStack.Computable> void options(B currentFrame, IVF1<B>... options) {
    options(currentFrame, arrayIterator(options));
  }
  
  <B extends VStack.Computable> void options(B currentFrame, Iterable<IVF1<B>> options);
  
  // if options list is empty, return null
  default <B extends VStack.Computable> void optionsOrReturn(B currentFrame, Cl<IVF1<B>> options) {
    if (empty(options))
      _return();
    else
      options(currentFrame, options);
  }
}