The second Project Euler problem - Even Fibonacci Numbers - is stated as follows - By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

```
// Generate a sequence thar produces the Fibonacci numbers.
// Fn = Fn-1 + Fn-2
let sequence = Seq.unfold (fun (a,b) -> Some( a+b, (b, a+b) ) ) (0,1)
let sum =
sequence
// Keep even numbers.
|> Seq.filter (fun n -> n % 2 = 0)
// Only take the values below four million.
|> Seq.takeWhile (fun n -> n < 4000000)
// Return the sum of the elements in the sequence.
|> Seq.sum
printfn "%i" sum
```