SICP习题2.17-2.18

(define (last-pair items)
  (cond ((or (null? items) (null? (cdr items))) items)
        (else (last-pair (cdr items)))))
(define (reverse items)
  (cond ((null? items) items)
        (else (append (reverse (cdr items)) (list (car items))))))
(define (append list1 list2)
  (cond ((null? list1) list2)
        (else (cons (car list1) (append (cdr list1) list2)))))
(define l1 (list 1 2 3 4 5))
(define l2 (list 6 7 8))