luhnDouble :: Int -> Int
    luhnDouble x | x * 2 > 9 = (x * 2) - 9
                 | otherwise = x * 2
luhnDouble :: Int -> Int
luhnDouble x | x * 2 > 9 = (x * 2) - 9
                 | otherwise = x * 2
luhnDouble x | x * 2 > 9 = (x * 2) - 9
                 | otherwise = x * 2