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type X =
    | I of int
    | S of string

let rec (|Nums|_|) l =
    match l with
    | [] -> None
    | (I x)::t -> 
        match t with 
        | Nums(xs, rest) -> Some(x :: xs, rest)
        | _ -> Some([x], t)
    | _ -> None

let stuff = [ I 1; I 2; I 3; S "four" ]
let things = [ S "a"; S "b"; I 3 ]
let leadingInts l =
    match l with
    | Nums(xs, rest) -> printfn "%A   ---   %A" xs rest
    | _ -> printfn "no leading ints"
leadingInts stuff
leadingInts things