Those three heuristics seem to work for the puzzles on the site, due to a limitation of the site's puzzle generator. However, there exist puzzles of this type w

Those three heuristics seem to work for the puzzles on the site, due to a limitation of the site's puzzle generator. However, there exist puzzles of this type which cannot be solved using only those three heuristics. Take this one, for instance: - O - - - - X - - O X - - - - - - - O - X - O - - - - - - O X - O - X - This puzzle has the unique solution shown below, but none of your three heuristics will make any progress on it. O O X X O X X X O O X O X O O X O X O X X O O X O X X O X O X O O X X O One of the ways you can solve this is to note that the square in the upper-left corner must be O. This is because if it were X, then there would be three adjacent Os in the first column. This insight is due to calebcjh@gmail.com.