diff --git a/fol.lisp b/fol.lisp
index 7e2a8fd..bc93296 100644
--- a/fol.lisp
+++ b/fol.lisp
@@ -2,11 +2,12 @@
   (and (symbolp x)
        (equal (char (symbol-name x) 0) #\?)))
 
-(defun length-form (form)
-  (if (> (length form) 0)
+(defun length-form (form &optional (n 0))
+  (if (not (null form))
       (if (atom (car form))
           (if (member (car form) '(implies and or equiv))
-              (+ 1 (length-form (cdr form)))
-              (length-form (cdr form)))
-          (+ (length-form (car form)) (length-form (cdr form))))
-      0))
+              (length-form (cdr form) (+ n 1))
+              (length-form (cdr form) n))
+          (progn (setf n (+ n (length-form (car form))))
+                 (length-form (cdr form) n)))
+      n))