1 (defun div-interval (x y)
2   (if (and
3        (>= (upper-bound y) 0)
4        (<= (lower-bound y) 0))
5       (error "Denominator spans zero")
6       (mul-interval
7        x
8        (make-interval (/ 1.0 (upper-bound y))
9 		      (/ 1.0 (lower-bound y))))))