anchor.diff:                                            (8 x x x x 2 x 9 x)
                                             (x x 4 5 7 x 3 x 2)))
   
  +(defparameter *20-minutes/1506/facile* #2A((3 x 5 8 4 x x x 1)
  +                                           (7 x x x x x 5 x x)
  +                                           (4 x 1 6 3 x 8 x x)
  +                                           (8 x x x x x 3 5 7)
  +                                           (x x 3 x x x 9 x x)
  +                                           (6 1 9 x x x x x 8)
  +                                           (x x 8 x 6 2 4 x 5)
  +                                           (x x 7 x x x x x 2)
  +                                           (2 x x x 5 1 7 x 3)))
  +
   (defparameter *metrofrance/694/moyen* #2A((x x 1 2 x x x x 8)
                                             (x x x x 5 1 x x 3)
                                             (x 7 x x x x 6 x 1)
  
positive.code:                                            (x x x x 6 x x x 1)
                                             (x 6 8 x x 9 x 5 7)))
  
  
  
  
  
  
  (dolist (sudoku (list
                   *royco-minut-soup*
                   *metrofrance/694/moyen*
                   *20-minutes/1499/facile*
                   *20-minutes/1501/difficile*
                   *20-minutes/1502/expert*
                   *metrofrance/696/facile*
                   *20-minutes/1505/moyen*
                   ))
    (multiple-value-bind (solutions tries) (sudoku-solver sudoku)
      (terpri)
      (sudoku-print sudoku)
      (format t "  has ~D solution~:*~P,~%  found in ~D tries.~2%" (length solutions) tries)
      (map nil 'sudoku-print solutions)))
  
  
  
negative.code:   sudoku)
  
  
  (defun sudoku-count-empty-slots (sudoku)
    (let ((empty-count 0))
      (for-each-slot (slot sudoku)
                     (when (emptyp slot)
                       (incf empty-count)))
      empty-count))
  
  ;;----------------------------------------------------------------------
  
  (defparameter *royco-minut-soup* #2A((x x x 8 x 4 2 x x)
  