sclass BStack > VStack {
  // alternative to backtrack to
  IF0<BStack> alternative;
  
  // steps to undo before switching to the alternative
  L<AutoCloseable> undos;
  
  BStack cloneStack() {
    new BStack s;
    s.stack = shallowCloneElements(stack);
    ret s;
  }
  
  void addUndo(AutoCloseable undo) {
    if (undo == null) ret;
    if (undos == null) undos = new L;
    undos.add(undo);
  }
  
  BStack addAlternative(IVF1<BStack> action) {
    BStack clonedStack = cloneStack();
    alternative = -> {
      action.get(clonedStack);
      ret clonedStack;
    };
    ret clonedStack;
  }
  
  BStack backtrack() {
    if (undos != null)
      for (undo : reversed(getAndClearList(undos)))
        undo.close();
    ret alternative?!;
  }
}