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 function, IVF1<B>... options) {
    options(function, arrayIterator(options));
  }
  
  <B extends VStack.Computable> void options(B function, Iterable<IVF1<B>> options);
}