Verifying ISSN's Check digit using Lisp

issn.lisp

(defun check-issn (str)
  " Parse a string of 8 digits in the format NNNN-NNNN representing an
    ISSN number. The last N can be an X representing 10. "
  (let ((digits (remove nil (loop for char across str
				  collect (cond ((equal char #\X) 10)
						((equal char #\-) nil)
						(t (parse-integer (string char))))))))
    (if (not (equal (length digits) 8))
	nil
	(equal 0 (mod (loop for i from 8 downto 1
			    for j in digits 
			    summing (* j i)) 11)))))