Exercise 2.28.  Write a procedure fringe that takes as argument a tree 
(represented as a list) and returns a list whose elements are all the 
leaves of the tree arranged in left-to-right order. For example,

(define x (list (list 1 2) (list 3 4)))

(fringe x)
(1 2 3 4)

(fringe (list x x))
(1 2 3 4 1 2 3 4)

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(define (fringe tree)
  (cond ((null? tree) nil)
        ((not (pair? tree)) (list tree))
        ((append (fringe (car tree)) (fringe (cdr tree))))))