sicp exercise 2.47

;; Exercise 2.47.  Here are two possible constructors for frames:

;; (define (make-frame origin edge1 edge2)
;;   (list origin edge1 edge2))

;; (define (make-frame origin edge1 edge2)
;;   (cons origin (cons edge1 edge2)))

;; For each constructor supply the appropriate selectors to produce an implementation for frames.

;; implementation 1

(define (make-frame origin edge1 edge2)
  (list origin edge1 edge2))
(define (origin-frame frame) (car frame))
(define (edge1-frame frame)  (cadr frame))
(define (edge2-frame frame)  (caddr frame))

;; implementation 2

(define (make-frame origin edge1 edge2)
  (cons origin (cons edge1 edge2)))
(define (origin-frame frame) (car frame))
(define (edge1-frame frame)  (cadr frame))
(define (edge2-frame frame)  (cddr frame))