Brand new difficulty level in Solo. The other day Gareth and I

Brand new difficulty level in Solo. The other day Gareth and I
independently discovered an advanced reasoning technique in Map, and
then it occurred to me that since Solo can also be considered as a
graph-colouring game the same technique ought to be applicable. And
it is; so here's a new difficulty level, `Extreme', which sits just
above Advanced. Grids graded `Extreme' by new-Solo will of course
fall into old-Solo's `Unreasonable' category (since they're not
soluble using the old set of non-recursive methods). A brief and
unscientific experiment suggests that about one in six Unreasonable
grids generated by old-Solo are classified Extreme by the new
solver; so the remaining Unreasonable mode (now containing a subset
of the grids it used to) hasn't actually become much harder.

[originally from svn r6209]