(define (a-op-b? op expr)
  (and (list? expr)
       (= (length expr) 3)
       (symbol? (car expr))
       (eq? op (car expr))
       (or (number? (cadr expr))
           (symbol? (cadr expr))
           (list? (cadr expr)))
       (or (number? (caddr expr))
           (symbol? (caddr expr))
           (list? (caddr expr)))))

(define (a*b? expr)
  (a-op-b? '* expr))

((a*b? expr)
         (let ((a (cadr expr))
               (b (caddr expr)))
           (list '+
                 (list '* (derivative a) b)
                 (list '* a (derivative b)))))