Aha, here's a nice easy way to generate really hard puzzles. Added

[sgt/puzzles] / puzzles.but

diff --git a/puzzles.but b/puzzles.but
index 1e6c0fa..7d90f43 100644 (file)
--- a/puzzles.but
+++ b/puzzles.but
@@ -691,9 +691,10 @@ particular, on difficulty levels \q{Trivial} and \q{Basic} there
 will be a square you can fill in with a single number at all times,
 whereas at \q{Intermediate} level and beyond you will have to make
 partial deductions about the \e{set} of squares a number could be in
-(or the set of numbers that could be in a square). None of the
-difficulty levels generated by this program ever requires making a
-guess and backtracking if it turns out to be wrong.
+(or the set of numbers that could be in a square). At
+\q{Unreasonable} level, even this is not enough, and you will
+eventually have to make a guess, and then backtrack if it turns out
+to be wrong.
 
 Generating difficult puzzles is itself difficult: if you select
 \q{Intermediate} or \q{Advanced} difficulty, Solo may have to make
@@ -738,6 +739,8 @@ parameters:
 
 \b \cq{da} for Advanced difficulty level
 
+\b \cq{du} for Unreasonable difficulty level
+
 So, for example, you can make Solo generate asymmetric 3x4 grids by
 running \cq{solo 3x4a}, or 4-way rotationally symmetric 2x3 grids by
 running \cq{solo 2x3r4}, or \q{Advanced}-level 2x3 grids by running