まず
(define (make-frame origin edge1 edge2) (list origin edge1 edge2))
に対する選択子がこれ。
(define (get-origin frame) (car frame)) (define (get-edge1 frame) (cadr frame)) (define (get-edge2 frame) (caddr frame))
次に
(define (make-frame origin edge1 edge2) (cons origin (cons edge1 edge2)))
に対する選択子は
(define (get-origin frame) (car frame)) (define (get-edge1 frame) (cadr frame)) (define (get-edge2 frame) (cddr frame))