let tails xs = Seq.unfold (fun es -> match es with [] -> None | _::er ->  Some(es,er)) xs

let rec combinations n xs =
    match n with
        | 0 -> Seq.singleton []
        | _ -> seq {for (y :: yr) in tails xs do
                    yield! seq {for rest in combinations (n-1) yr do yield y :: rest}}

[<EntryPoint>]
let main args =
    [0..20]
    |> combinations 8
    |> Seq.toList
    |> printf "%A"
    0