&gt; For Sudoku puzzles generated for humans<p>I agree that there&#x27;s a significant difficulty jump, when that algorithm fails to suffice, and I think it&#x27;s fair to say that&#x27;s the &quot;hard&quot; jump. But I think a lot of puzzles intended for humans are &quot;hard&quot;, by this definition.<p>Teaching a computer that special-case rule might not make sense for speedy computer solving, but I think it would make sense for telling a human &quot;here&#x27;s how you could have found this number&quot;.