연습문제 2.33

(define (map_test p sequence)
  (accumulate (lambda (x y) (cons (p x) y)) nil sequence))

(define (append_test seq1 seq2)
  (accumulate cons seq2 seq1))

(define (length_test sequence)
  (accumulate
   (lambda (x y)
     (+ 1 y))
   0 sequence))