(require 'cl)
(defun quicksort (xs)
  (if (null xs)
      nil
    (let* ((pivot (car xs))
           (smaller (remove-if-not #'(lambda (x) (< x pivot)) (cdr xs)))
           (greater (remove-if-not #'(lambda (x) (>= x pivot)) (cdr xs))))
      (append (quicksort smaller)
              (list pivot)
              (quicksort greater)))))