The sum of all even Fibonacci numbers below four million is easily found with a recursive function and a switch.

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.

```
let fib target =
let rec next x y sum =
match (y mod 2 == 0, y < target) with
| (true, true) -> next y (x + y) (sum + y)
| (false,true) -> next y (x + y) sum
| _ -> sum in
next 1 1 0
let _ = print_int (fib 4000000)
```

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