> A direct implementation without using a library is interesting, thank
> you. I
> refrained from doing that, because I thought that Control.Zipper
> would actually
> do this for me. Actually, I was pretty successful with using
> Control.Zipper to
> change node labels, but failed doing more complicated stuff.
>
Isn't that a strong indicator that zippers are an improper abstraction
for your purpose? Perhaps after rolling your own implementation you can
more easily discover how to represent the algrorithm as a zipper.

Olaf

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