[xsl] Sudoku solver - complete

For those of you interested in this I've now finished performance
tuning my Sudoku solution.

I'm quite pleased with it - boards that were taking 15mins+ now take
less than a few seconds.  The most pleasing aspect is the way it
selects the allowed values for a cell - it's much better at detecting
when there's only one possible value, and the way it works through the
board.

I'm also making much more use of subsequence() - several seconds were
shaved off after constructing new sequences from existings sequences
using this function.

I'm satisfied that it's complete now, but if anyone can improve it or
break it, please let me know

cheers
andrew