1 (define (make-rat n d)
2   (cons n d))
3 (define (numer x)
4   (let ((g (gcd (car x) (cdr x))))
5     (/ (car x) g)))
6 (define (denom x)
7   (let ((g (gcd (car x) (cdr x))))
8     (/ (cdr x) g)))
9