let nPi =     
    let piSeq = seq{
        yield 3.0
        let rec pi start sign = seq{
            yield (sign * 4.0) / ( start * (start + 1.0) * (start + 2.0))
            yield! pi (start + 2.0) (sign * -1.0)
        }

        yield! pi 2.0 1.0
    }
    piSeq |> Seq.take 10 |> Seq.sum

// using unfold
let p' = 
    let calc start sign = (sign * 4.0) / ( start * (start + 1.0) * (start + 2.0))

    Seq.unfold (fun (start, sign) -> Some ( (calc start sign), (start + 2.0, sign * -1.0) )) (2.0, 1.0)
    |> Seq.take 10 
    |> Seq.sum 
    |> (+) 3.0