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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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