/** An operation to perform on a list. */
  interface ListFrobber {
    void perform(List<Integer> list);
  }

  static ListFrobber add(int element) {
    return new ListFrobber() {
      @Override
      public void perform(List<Integer> list) {
        list.add(0, element);
      }
    };
  }

  static ListFrobber remove() {
    return new ListFrobber() {
      @Override