(define (div-interval x y)
  (if (or (and (<= 0 (lower-bound x)) (>= 0 (upper-bound x)))
          (and (>= 0 (lower-bound x)) (<= 0 (upper-bound x)))
          (and (<= 0 (lower-bound y)) (>= 0 (upper-bound y)))
          (and (>= 0 (lower-bound y)) (<= 0 (upper-bound y))))
      "division by zero"
      (mul-interval x
                    (make-interval
                     (/ 1.0 (upper-bound y))
                     (/ 1.0 (lower-bound y))))))