; Terribly tentative derivative function.
(lambda (e v) (let deriv ((e e)) (if (number? e) 0
  (if (pair? e) (let ((head (car e)))
     (if (or (eq? head '+) (eq? head '-)) `(,head ,@(let ds ((el (cdr e))) 
         (if (null? el) '() (cons (deriv (car el)) (ds (cdr el))))))
     (if (eq? head '*) (if (not (= (length (cdr e)) 2)) 'po*
       `(+ (* ,(cadr e) ,(deriv (caddr e))) (* ,(deriv (cadr e)) ,(caddr e))))
     (if (eq? head 'exp) `(* ,(deriv (cadr e)) (exp ,(cadr e))) 'poX))))
   (if (symbol? e) (if (eq? e v) 1 0) 'poY)))))
 
 
((fileVal "deriv") '(+ (* r 3) (* s r) (- s 2)) 'r)