Dan's Thoughts Thinking somewhat carefully

Exercise 2.28 of SICP

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)