(defun is-prime (n &optional (d (- n 1)))
  (if (/= n 1) (or (= d 1)
          (and (/= (rem n d) 0)
               (is-prime  n (- d 1)))) ()))

(defun prime-factors-helper (x n)
   (if (is-prime x)
       (list x)
       (if (is-prime n)
            (if (AND (= (mod x n) 0) (<= n (/ x 2)))
                (cons n (prime-factors-helper (/ x n) n))
                (prime-factors-helper x (+ 1 n)))
            (prime-factors-helper x (+ 1 n)))))

(defun prime-factors (x)
    (prime-factors-helper x 2))

CL-USER 53 > (prime-factors 512)
 (2 2 2 2 2 2 2 2 2)

 CL-USER 54 > (prime-factors 123456789)

 Stack overflow (stack size 261120).
   1 (abort) Return to level 0.
   2 Return to top loop level 0.

 Type :b for backtrace or :c <option number> to proceed.
 Type :bug-form "<subject>" for a bug report template or :? for other options