**Even Fibonacci numbers**

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

mfib :: Int -> Integer
mfib = (map fib [0 ..] !!)
where fib 0 = 0
fib 1 = 1
fib n = mfib (n-2) + mfib (n-1)
sum [ y | x <- [1..1000], let y = mfib(x), y < 4000000, y `mod` 2 == 0 ]

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