SICP 問題 2.10

;; y が0をまたがる区間の時はエラーを返す
(define (div-interval x y)
  (if (> 0 (* (lower-bound y) (upper-bound y)))
      (error "error")
      (mul-interval x
                    (make-interval (/ (upper-bound y))
                                   (/ (lower-bound y))))))