let rec factorial(n : int) (mem : bigint) =
    match n with
    | 0 | 1 -> printfn "%A" mem
    | _ ->  factorial (n - 1) (mem * bigint(n))


let BigFactorial(numero,mesaje)=
    Async.FromContinuations(fun (cont,error,cancelation) ->
                                printfn "begin number: %s" mesaje
                                factorial numero 1I |>ignore
                                printfn "begin number: %s ." mesaje
                                cont())

//this doesn't works in async way...I get the 30M result first and then run the 10M
Async.RunSynchronously(async{
    printfn "Start!!..."
    do! BigFactorial(30000,"30M")
    do! BigFactorial(10000, "10M")
    printfn "End!!..."
})



//this works nicely
let task1 = async{
    printfn "begin"
    do! BigFactorial(30000,"30")
    printfn "end..."
}

let task2 = async{
    printfn "begin"
    do! BigFactorial(10000,"10")
    printfn "end!!..."
}


Async.RunSynchronously(Async.Parallel[task1;task2])