2> (list-zipper '(a (b (c d))))
#<zip (a (b (c d)))>
3> (down (right (down #2)))
#<zip b>
4> (replace #3 'XX)
#<zip x>
5> (root #4)
#<zip (a (XX (c d)))>
6> #2
#<zip (a (b (c d)))>