adequately hard one couldn't be found, but we must never return a
puzzle harder than the user asked for, and we _certainly_ mustn't
return one that we aren't even sure has a unique solution.
However, when I attempted to implement this rule, it turned out that
an attempt to generate a 15x15 Easy puzzle didn't terminate before I
got bored and killed it. I'm therefore retiring the Easy difficulty
level on the grounds that it's not a wide enough band to be useful.
I've renamed Hard to Normal, and (while I was there) renamed
Recursive to Unreasonable in line with my normal practice.
[originally from svn r7309]
