lesson-lisp/lib/lists.scm

(define (caar x) (car (car x)))
(define (cadr x) (car (cdr x)))
(define (cdar x) (cdr (car x)))
(define (cddr x) (cdr (cdr x)))

(define (caaar x) (car (car (car x))))
(define (caadr x) (car (car (cdr x))))
(define (cadar x) (car (cdr (car x))))
(define (caddr x) (car (cdr (cdr x))))
(define (cdaar x) (cdr (car (car x))))
(define (cdadr x) (cdr (car (cdr x))))
(define (cddar x) (cdr (cdr (car x))))
(define (cdddr x) (cdr (cdr (cdr x))))

(define (caaaar x) (car (car (car (car x)))))
(define (caaadr x) (car (car (car (cdr x)))))
(define (caadar x) (car (car (cdr (car x)))))
(define (caaddr x) (car (car (cdr (cdr x)))))
(define (cadaar x) (car (cdr (car (car x)))))
(define (cadadr x) (car (cdr (car (cdr x)))))
(define (caddar x) (car (cdr (cdr (car x)))))
(define (cadddr x) (car (cdr (cdr (cdr x)))))
(define (cdaaar x) (cdr (car (car (car x)))))
(define (cdaadr x) (cdr (car (car (cdr x)))))
(define (cdadar x) (cdr (car (cdr (car x)))))
(define (cdaddr x) (cdr (car (cdr (cdr x)))))
(define (cddaar x) (cdr (cdr (car (car x)))))
(define (cddadr x) (cdr (cdr (car (cdr x)))))
(define (cdddar x) (cdr (cdr (cdr (car x)))))
(define (cddddr x) (cdr (cdr (cdr (cdr x)))))

(define (list-tail x k)
  (if (zero? k)
      x
      (list-tail (cdr x) (- k 1))))

(define (list-ref x k) (car (list-tail x k)))