>>543
色々いじっていたらこんなになった
(lambda (xs)
(define (foo x y z)
(if (= x y) (cons x z) (foo x (- y 1) (cons y z))))
(let fold
([f (lambda (x z) (if (pair? x) (foo (car x) (cdr x) z) (cons x z)))]
[z '()]
[xs (reverse xs)])
(if (null? xs) z (fold f (f (car xs) z) (cdr xs)))))