(defun keepfirst (n list &optional (res nil))
   (cond ((or (= n 0) (null list))(reverse res))
         (t (keepfirst (- n 1) (rest list) (cons (first list) res)))
   )
 )

(defun inbetween (pos1 pos2 list)
  (keepfirst (- pos2 (1+ pos1)) (nthcdr (1+ pos1) list))
)

(defun swap (pos1 pos2 list)
  (append (keepfirst pos1 list)
          (list (nth pos2 list)) 
   (inbetween pos1 pos2 list)
          (list (nth pos1 list))
   (nthcdr (1+ pos2) list)) 
)

;; (load "swap")
;; (setf mylist '(a b c d e f))
;; (swap 0 3 mylist)
;; (D B C A E F)
;; (swap 2 3 mylist)
;; (A B D C E F)
;; (swap 0 2  mylist)
;; (C B A D E F)
;; (swap 0 1  mylist)
;; (B A C D E F)