+ * * actually, we seem to be mostly sensible already now. I
+ * want more choice over the type of main block and location
+ * of the exit/target, and I think I probably ought to give
+ * up on compactness and just bite the bullet and have the
+ * target area right outside the main wall, but mostly I
+ * think it's OK.
+ * * but adjust the presets, because 8x6 is _slow_ to
+ * generate.
+ * * also, introduce a difficulty scheme, in the form of
+ * limiting the minimum move count. This is obviously
+ * sensible, because it also speeds up generation since the
+ * solver can bomb out once it hits that ceiling!
+ * + I was going to say I'd need to work out a minimum move
+ * count for each grid size, but actually I think not: if
+ * you ask for too few moves, it just has to remove still
+ * more singletons, until at move count 1 you end up with
+ * nothing in your way at all and it SERVES YOU RIGHT!