(define (fringe tree)
  ; right-fringe contains the fringe of the tree excluding node which is all
  ; the leaves in node's right sibling and node's ancestors' right siblings.
  ; iter returns the fringe excluding leafs from node's left siblings and nodes
  ; ancestors' left siblings.
  (define (iter node right-fringe)
    (cond ((null? node) right-fringe)
          ((not (pair? node)) ; leaf
            (cons node right-fringe))
          (else
            (iter (car node) (iter (cdr node) right-fringe)))))
  (iter tree '()))

; 1 ]=> (define x (list (list 1 2) (list 3 4)))
; 
; ;Value: x
; 
; 1 ]=> (fringe x)
; 
; ;Value: (1 2 3 4)
; 
; 1 ]=> (define y (list 1 2 (list 3 (list 4 5) 6) 7 8 (list (list 9))))
; 
; ;Value: y
; 
; 1 ]=> y
; 
; ;Value: (1 2 (3 (4 5) 6) 7 8 ((9)))
; 
; 1 ]=> (fringe y)
; 
; ;Value: (1 2 3 4 5 6 7 8 9)