interface IBStack<A> {
  // declare a choice point for later backtracking
  <B extends VStack.Computable> void options(B function, IVF1<B>... options);
  <B extends VStack.Computable> void options(B function, Iterable<IVF1<B>> options);
  
  // record an undoable action for backtracking
  void addUndo aka temp(AutoCloseable undo);
  
  A nextResult();
  A nextResultWithPrintStruct(long maxSteps);
}