open Microsoft.FSharp.Collections.LazyList
let rec merge a b =
    let cons' x f = LazyList.consDelayed x f
    match a, b with
    | (Cons(x, xs) as left), (Cons(y, ys) as right) ->
        if x < y then cons' x (fun () -> merge xs right)
        elif x > y then cons' y (fun () -> merge left ys)
        else cons' x (fun () -> cons' y (fun() -> merge xs ys))
    | Nil, ys -> ys
    | xs, Nil -> xs