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))