1 (define (deriv exp var)
2   (cond ((number? exp) 0)
3 	((variable? exp)
4 	 (if (same-variable? exp var) 1 0))
5 	((sum? exp)
6 	 (make-sum (deriv (addend exp) var)
7 		   (deriv (augend exp) var)))
8 	((product? exp)
9 	 (make-sum
10 	  (make-product (multiplier exp)
11 			(deriv (multiplicand exp) var))
12 	  (make-product (deriv (multiplier exp) var)
13 			(multiplicand exp))))
14 	(else
15 	 (error "unknown expression type -- DERIV" exp))))