let sum xs =
    let rec loop n xs' =
        match xs' with
        | []     -> n
        | hd::tl -> loop (hd + n) tl
    match xs with
    | []     -> 0
    | hd::tl -> loop hd tl

[<EntryPoint>]
let main argv =
    printfn "%A" (sum [0..100])
    0