#lang sicp

(define (list? x)
  (or (null? x) (pair? x)))

(define (fringe tree)
  (if (null? tree)
      '()
      (let* ((head (car tree))
             (tail (cdr tree))
             (h-list (if (list? head) ; if it's a subtree
                         (fringe head) ; turn it into a list
                                   ; otherwise, just make a
                                   ; one-element list
                         (list head))))
    (append h-list (fringe tail)))))