luhn :: Int -> Int -> Int -> Int -> Bool
    luhn a b c d = luhnTotal `mod` 10 == 0
                   where luhnTotal = luhnDouble a + b + luhnDouble c + d
luhn :: Int -> Int -> Int -> Int -> Bool
luhn a b c d = luhnTotal `mod` 10 == 0
                   where luhnTotal = luhnDouble a + b + luhnDouble c + d
luhn a b c d = luhnTotal `mod` 10 == 0
                   where luhnTotal = luhnDouble a + b + luhnDouble c + d