;;; Function that returns all possible choices of K elements from a
;;; given list L.  Solution courtesy of Martin Buchmann, with cosmetic
;;; editions by JM.

(defun combination (k l)
  (cond
   ((< k 0) nil)
   ((= k 0) (list nil))
   ((> k (length l)) nil)
   (t (append (mapcar #'(lambda (x) (cons (first l) x))
		      (combination (1- k) (rest l)))
	      (combination k (rest l))
	      )
      )
   )
  )

;;; Alternative solution sent to me by Wojciech Gac on 2014-01-17
;;; Just indentation editing by JM.

(defun combination (n lst)
  (cond
   ((zerop n) nil)
   ((= n 1) (mapcar #'list lst))
   (t (mapcan #'(lambda (x)
		  (mapcar #'(lambda (y) (append (list x) y))
			  (combination (1- n) (setf lst (delete x lst)))))
	      lst))
   )
  )