I should have read the comment before the code. I was just coming to the conclusion that the code could not solve the harder sudokus. The wikipedia article on

I should have read the comment before the code. I was just coming to the conclusion that the code could not solve the harder sudokus. The wikipedia article on "Algorithmics of Sudoku" is a start for tips. There is a Scientific American article that has a good overview of the problem... http://www.cs.utexas.edu/~kuipers/readings/Sudoku-sciam-06.pdf http://www.cs.utexas.edu/~kuipers/readings/Sudoku-sciam-06.p... Don't google for Lispers writing solvers. They will make you feel all bloated.