Exercise 2.33.  Fill in the missing expressions to complete the 
following definitions of some basic list-manipulation operations as 
accumulations:

(define (map p sequence)
  (accumulate (lambda (x y) <??>) nil sequence))
(define (append seq1 seq2)
  (accumulate cons <??> <??>))
(define (length sequence)
  (accumulate <??> 0 sequence))

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

(define (append seq1 seq2)
  (accumulate cons seq2 seq1))

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