And with the Seq module (left at end), combined with the pa_monad 
extention, one can rewrite find_comb as

let find_comb p0 p1 =
  perform with Seq in
    i0 <-- range p0;
    i1 <-- range p0;
    i2 <-- range p1;
    guard (test i0 i1 i2);
    return (i0, i1, i2)

I do believe that is more readable :-)

Jacques

Pietro Abate wrote:
> I'm not sure this is exactly what you want... but I think it's a good
> starting point to look at for this kind of problems. Making it lazy is
> just a matter of changing the definition of the modules Seq.
>
> module Seq =
>     struct
>         let mzero = []
>         let return a = [a]
>         let bind m f = List.flatten (List.map f m)
>         let mplus = List.append
>         let guard b = if b then return () else mzero
>     end
>         ;;
>
> let range n =
>   let rec aux i l =
>       if i = 0 then l else i::(aux (i-1) l)
>   in List.rev ( aux n [] )
> ;;
>
> let test _ _ _ = true ;;
>
> let find_comb p0 p1 =
>     Seq.bind (range p0) (fun i0 ->
>         Seq.bind (range p0) (fun i1 ->
>             Seq.bind (range p1) (fun i2 ->
>                 Seq.bind (Seq.guard (test i0 i1 i2)) (fun _ ->
>                     Seq.return (i0,i1,i2)
>                 )
>             )
>         )
>     )
> ;;
>
>