If you've found X of (say) length L and
order N, and then find a different Y of length L and order N, I can't
see any justification for the assumption that you can prune the entire
subtree below Y [...]

I do not prune the search tree. If I say I "forget" about Y,
then I do not mean "forget" all transforms that start with Y.
That would be a bad thing, of course.
--
Michiel Boland <boland@sci.kun.nl>
University of Nijmegen
The Netherlands