Exercise 2.39.   Complete the following definitions of reverse (exercise 
2.18) in terms of fold-right and fold-left from exercise 2.38:

(define (reverse sequence)
  (fold-right (lambda (x y) <??>) nil sequence))
(define (reverse sequence)
  (fold-left (lambda (x y) <??>) nil sequence))

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(define (reverse sequence)
  (fold-right (lambda (x y) (append y (list x))) nil sequence))

(define (reverse sequence)
  (fold-left (lambda (x y) (cons y x)) nil sequence))