flattenの定義
(define (flatten list) (let loop ((list1 (reverse list)) (list2 '())) (if (null? list1) list2 (if (list? (car list1)) (loop (cdr list1) (append (flatten (car list1)) list2)) (loop (cdr list1) (cons (car list1) list2))))))
(define (flatten list) (let loop ((list1 (reverse list)) (list2 '())) (if (null? list1) list2 (if (list? (car list1)) (loop (cdr list1) (append (flatten (car list1)) list2)) (loop (cdr list1) (cons (car list1) list2))))))