Project Euler 2 - Even Fibonacci Numbers - Solved with F#

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