; deriv-at-point : (num -> num) num num -> num
(define (deriv-at-point f x eps)
  (local ((define delta-x (* 2 eps))
	  (define delta-y (- (f (+ x eps)) (f (- x eps)))))
    (/ delta-y delta-x)))

; deriv : (num -> num) -> (num -> num)
(define (deriv f)
  (local ((define eps .001) ; an arbitrary small value
	  (define (f-deriv x)
	    (local ((define delta-x (* 2 eps))
		    (define delta-y (- (f (+ x eps)) (f (- x eps)))))
	      (/ delta-y delta-x))))
    f-deriv))