Aha, here's a nice easy way to generate really hard puzzles. Added
the missing fifth difficulty level to Solo: `Unreasonable', in which
even set-based reasoning is insufficient and there's no alternative
but to guess a number and backtrack if it didn't work. (Solutions
are still guaranteed unique, however.) In fact it now seems to take
less time to generate a puzzle of this grade than `Advanced'!

[originally from svn r5756]