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)
(define (fringe x)
(cond ((null? x) '())
((not (pair? x))
(list x))
(else
(append (fringe (car x)) (fringe (cdr x))))))
> (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)