Reduce recursivamente a un dígito un entero mediante suma.
reduceADigito :: Int -> Int
reduceADigito x
| x < 10 = x
| otherwise = reduceADigito $ sumaDigitos $ obtenDigitos x
sumaDigitos :: [Int] -> Int
sumaDigitos [] = 0
sumaDigitos (x:xs) = x + sumaDigitos xs
obtenDigitos :: Int -> [Int]
obtenDigitos 0 = []
obtenDigitos x = y : obtenDigitos ( ( x - y ) `div` 10 )
where y = x `mod` 10
