a Fibonacci with unfold up to 4mil for euler

fib.fs

let fib x =
    (0,0)
    |> Seq.unfold(fun (a,b) ->
        match a,b with
        | 0, 0 -> Some(1, (0, 1))
        | _ ->
            let c = a + b
            Some(c, (b,c))
    )
    |> Seq.indexed
    |> Seq.map(fun (i,x) -> i+1, x)
    |> Seq.takeWhile(fun (i,v) -> i <= x && v < 4_000_000)
    |> Seq.last