Problem 28

p28 :: Integer -> Integer
p28 limit = 1 + (sum $ concat $ map (\x -> p28' limit 1 x) [2,4,6,8])
  where 
    p28' limit a b = takeWhile (\x -> x <= limit) $ p28'' a b
    p28'' a b = (a+b) : p28'' (a+b) (b+8)

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