1 (defun par1 (r1 r2)
2   (div-interval
3    (mul-interval r1 r2)
4    (add-interval r1 r2)))
5 (defun par2 (r1 r2)
6   (let ((one (make-interval 1 1)))
7     (div-interval
8      one
9      (add-interval (div-interval one r1)
10 		   (div-interval one r2)))))