Fix VVPs a bit more correctly

checking.lisp

@@ -22,9 +22,11 @@
 (defun fix-vvps (vvps constraint)
   (let ((variables (variables constraint)))
     (mapcar #'(lambda (var)
-                (find var vvps
-                      :key #'car
-                      :test #'equal))
+                (if (numberp var)
+                    var
+                    (find var vvps
+                          :key #'car
+                          :test #'equal)))
             variables)))
 
 (defmethod check-constraint ((constraint <constraint-extension>) vvps &key &allow-other-keys)