--2
--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.


fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
euler2 = sum $  filter (even) $ takeWhile(< 4000000) fibs

