(define (sqrt2 n d)
    (sqrt2-help n d 1 0))

(define (sqrt2-help n d guess prevguess)
    (if (< (diff guess prevguess) (* d prevguess))
	guess
	(sqrt2-help n d (improve-guess n guess) guess)))

(define (diff a b)
    (abs (- a b)))

(define (improve-guess n guess)
    (average guess (/ n guess)))

(define (average a b)
    (/ (+ a b) 2))

yeah, i guess. it's d for delta, but it really means that the new guess needs to be less than a dth of the previous one. dunno what to call that.