help a newbie with Lazy

Hi. I am trying to teach myself OCaml, so I was starting with the examples from John Hughes' paper on functional design. In particular I was trying to get his algorithm for square root to work -

let next y x = (x +. y /. x) /. 2.;;
let rec repeat f x = x::repeat f (f x);;
let rec within eps (a::b::rest) = if abs_float(a -. b)<eps then b else within eps (b::rest);;
let newton_sqrt x0 eps y = within eps (repeat (next y) x0);;

newton_sqrt 4. 1e-5 17.;;

This fails, because I guess OCaml is not lazy by default. I suppose I need to apply a 'lazy' and a 'Lazy.force' in here somewhere, but I'm not sure where. I made various attempts without success. Can anyone give me a hint?

Regards,
John