SICP 問題 2.28
(define x (list (list 1 2) (list 3 4))) (define (fringe l) (cond ((null? l) nil) ((pair? (car l)) (append (fringe (car l)) (fringe (cdr l)))) (else (cons (car l) (fringe (cdr l))))))
gosh> (fringe x) (1 2 3 4) gosh> (fringe (list x x)) (1 2 3 4 1 2 3 4)