returns the check digit of the card number.

def get_check_digit(unchecked):
    """returns the check digit of the card number."""
    digits = digits_of(unchecked)
    checksum = sum(even_digits(unchecked)) + sum([
        sum(digits_of(2 * d)) for d in odd_digits(unchecked)])
    return 9 * checksum % 10