Hi Graham,

There is one implementation here:

http://code.haskell.org/yi/Data/Tree/

I wrote it for Yi but it is quite general. It is a pity that we don't
have it in the standard libraries. It is not completely tested but it
seems to work for me.

Thank very much, Krasimir! I agree, it would be a good addition to the standard library.

I've added a link to your implementation from the Zipper page in the Haskell wiki.

Regards,
Graham