Luhn algorithm in elisp

luhn-check.el

(defun luhn-sum (list n)
  (if (null list)
      0
    (+ (let ((x (car list)))
	     (if (= 1 (mod n 2))
		 (let ((y (* 2 x)))
		   (if (> y 9) 
		       (+ 1 (mod y 10)) 
		     y))
	       x))
       (luhn-sum (cdr list) (+ 1 n)))
    )
)

(defun card-no-str-to-num (card-no)
  (mapcar (lambda (x) (string-to-number x 10)) (cdr (reverse (cdr (split-string card-no "")))))
)

(defun luhn-check (card-no)
  (eq 0 (mod (luhn-sum (card-no-str-to-num card-no) 0) 10))
)

(luhn-check "49927398716")