Producing k-combinations of a set of elements in F#

kCombinations.fsx

// assuming elements in a set do not repeat
let rec kCombinations k (set: 'a list) =
    match k with
    | 1 -> set |> List.map (fun x -> [x])
    | _ ->
        match set with
        | [] -> []
        | head::tail ->
            (tail |> kCombinations (k - 1) |> List.map (fun x -> head::x)) 
            @ (tail |> kCombinations k)

printfn "%A" ([1;2;3;4] |> kCombinations 2)

what about without repetition ?