type 'a llist = LNil | LCons of 'a * 'a llist lazy_t;;
let rec ltake n = function
 | LCons (a, ll) when n > 0 -> a::(ltake (n-1) (Lazy.force ll))
 | _ -> []
;;
let rec lfrom n = LCons (n, lazy (lfrom (n+1)));;
let rec lfilter f = function
 | LNil -> LNil
 | LCons (n, ll) ->
     if f n then LCons (n, lazy (lfilter f (Lazy.force ll)))
     else lfilter f (Lazy.force ll)
;;
let primes =
 let rec sieve = function
     LCons(p,nf) -> LCons(p, lazy (sieve (sift p (Lazy.force nf))))
   | LNil -> failwith "Impossible! Internal error."
 and sift p = lfilter (function n -> n mod p <> 0)
in sieve (lfrom 2)
;;

let times ll n = lmap (fun i -> i * n) ll;;

let rec merge xs ys = match xs, ys with
 | LCons (x, xr), LCons (y, yr) ->
     if x < y then LCons (x, lazy (merge (Lazy.force xr) ys))
     else if x > y then LCons (y, lazy (merge xs (Lazy.force yr)))
     else LCons (x, lazy (merge (Lazy.force xr) (Lazy.force yr)))
 | r, LNil | LNil, r -> r
;;

let hamming k =
 let pr = ltake k primes in
 let rec h = LCons (1, lazy (
   <TODO> )) in
 h
;;