}

  // use an enum to get easy serializability
  private enum CharAtFunction implements Function<String, Character> {
    AT0(0),
    AT1(1),
    AT2(2),
    AT3(3),
    AT4(4),
    AT5(5),
    ;

    final int index;

    CharAtFunction(int index) {
      this.index = index;
    }

    @Override
    public Character apply(String string) {

  }

  // use an enum to get easy serializability
  private enum CharAtFunction implements Function<String, Character> {
    AT0(0),
    AT1(1),
    AT2(2),
    AT3(3),
    AT4(4),
    AT5(5),
    ;

    final int index;

    CharAtFunction(int index) {
      this.index = index;
    }

    @Override
    public Character apply(String string) {