public Maybe<Zipper<A>> maybeModifyNext(UnaryOperator<A> operator){
  return Maybe.JustWhenTrue(!isEnd(),null);
}

public Maybe < Zipper < A > > maybeModifyNext ( UnaryOperator < A > operator ) { return Maybe . JustWhenTrue ( ! isEnd ( ) , null ) ; }
modifies the element after the focus position , if possible .
