Euler, ex. 2 F#.fs

(* Recursive way - this gives the nth fibonacci number *)
let rec fib n = if n < 2 then 1 else fib (n - 1) + fib (n - 2)
let fibonaccis = 0|> Seq.unfold (fun(x) -> Some(fib(x), x+1))
Seq.takeWhile (fun x -> x < 4000000) fibonaccis
             |> Seq.filter (fun x -> x % 2 = 0)
             |> Seq.fold (+) 0

(* Try putting the upper sequence limit into the sequence
   Something's wrong here: takes five minutes and the answer is wrong (1485607536) *)
let rec fib n = if n < 2 then 1 else fib (n - 1) + fib (n - 2)
let fibonaccis = Seq.unfold( fun(x) -> if x >= 4000000 then None else Some(fib(x), x+1) ) 0
fibonaccis |> Seq.filter (fun x -> x % 2 = 0)
           |> Seq.fold (+) 0