;;; 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))
	      )
      )
   )
  )