;;; $Id: nth-element.scm,v 1.1 2004/01/29 04:53:50 leavens Exp $
;;; AUTHOR: Dalei Li and Gary T. Leavens

(deftype nth-element (forall (t) (-> ((list-of t) number) t)))
(define nth-element
  (lambda (lst n)
    ;; ENSURES: Result is the nth, zero-based, element of lst
    (if (null? lst)
	(eopl:error 'nth-element
		    "List too short by ~s elements" (+ n 1))
	(if (zero? n)
	    (car lst)
	    (nth-element (cdr lst) (- n 1))))))