a non-recursive level above Easy, which therefore moves the
recursive Hard mode further up still. Play-testing suggests that in
fact Tricky is often _harder_ than the old Hard mode, since the
latter had limited depth of recursion and would therefore spot
complex deductions only if it happened to start a recursion on the
right square; Tricky may be limited in the sophistication of its
complex deductions, but it never misses one, so its puzzles tend to
be hard all over.
Also in this checkin, a new source file `nullfe.c', containing all
the annoying stub functions required to make command-line solvers
link successfully. James wrote this for (the new) lightupsolver, and
I've used it to simplify the other stand-alone solvers.
[originally from svn r6254]
