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) 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)