(defun qsort (input predicate)
  (if input
    (let* ((pivot (first input))
           (rest (rest input))
           (lesser (remove-if-not #'(lambda (x)
                                      (funcall predicate x pivot))
                                  rest))
           (greater (remove-if-not #'(lambda (x)
                                       (not (funcall predicate x pivot)))
                                   rest)))
      (append (qsort lesser predicate)
              (list pivot)
              (qsort greater predicate)))
    nil))
 
(format t "~a~%" (qsort '(9 8 5 6 7 2 3 1 4) #'<))

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