let duplicate(k,xs) =
    let rec wew(k,xs) =
        match (k,xs) with
        (_,LNil) -> LNil
        | (0,LCons(_,t)) -> wew(k,t())
        | (n,LCons(h,_)) -> LCons(h, function () -> wew(k-1,xs))
    in wew(k,xs);;