Works, but for some sudokus it runs out of stack space

sudoku.lispbm

@@ -24,6 +24,18 @@
     (9   nil 2   3   4   5   nil nil 8)
     (3   4   5   6   7   8   nil 1   2)))
 
+(define toSolve2
+  '(
+    (2   4   3   1   6   5   7 9   8)
+    (6   1   9   nil nil nil 2 4   5)
+    (5   8   7   nil 4   nil 1 nil 6)
+    (4   nil 6   nil 1   2   9 8   3)
+    (3   9   nil 8   nil 4   6 nil 2)
+    (7   nil nil 6   nil 3   4 nil 1)
+    (9   nil 2   5   nil 7   8 nil 8)
+    (8   nil nil 4   2   1   3 nil 9)
+    (1   nil nil nil nil 6   5 2   7)))
+
 (defun amount (x y)
   (foldr (lambda (z q) (if (eq x z) (+ q 1) q)) 0 y))
 
@@ -119,23 +131,40 @@
         (matrixToList (getSubMatrix m sr er sc ec)))
       'error))
 
+(defun printSudoku (m) (loopforeach i m (print i)))
+
 (defun solve (grid)
   (let ((empties (findEmpty grid))
         (size1 (foldr
                   (lambda (x ys)
                     (let ((e (match x
-                                (((?r) (?c)) (notPresent r c)))))
+                                (((?r) . (?c)) (notPresent grid r c)))))
                         (if (= 1 (length e))
-                            (cons (append e x) ys)
+                            (cons (list (car e) (car x) (cdr x)) ys)
                             ys)))
                   nil
+                  empties))
+        (others (foldr
+                  (lambda (x ys)
+                    (let ((e (match x
+                                (((?r) . (?c)) (notPresent grid r c)))))
+                            (cons (list e (car x) (cdr x)) ys)))
+                  nil
                   empties)))
     (if (eq empties nil)
-        grid
+        (if (valid grid)
+            grid
+            'not-valid)
         (if (eq size1 nil)
-            () ; TODO: Empty
+            (match (car others)
+                   (((? es) (?r) (?c))
+                    (foldr
+                     (lambda (e ys) (match (solve (place grid r c e))
+                                        ('not-valid ys)
+                                        (xs xs)))
+                     nil
+                     es)))
             (solve (foldr (lambda (x gs)
                      (match x
-                        ((?e) (?r) (?c)) (place g r c e)))
+                        (((?e) (?r) (?c)) (place gs r c e))))
-                   grid size1))
+                   grid size1))))))
-           ))))