SICP Exercise 2.39
The procedure reverse-fl is simply consing the elemets.
(define (reverse-fl sequence) (fold-left (lambda (x y) (cons y x)) nil sequence))
For reverse-fr, simply using cns ruins the structure of the list. Using append is a better approach.
(define (reverse-fr sequence) (fold-right (lambda (x y) (append y (list x))) nil sequence))