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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 luhn(n):
 """Validate that a string made of numeric characters verify Luhn test. Used by siret validator. from http://rosettacode.org/wiki/Luhn_test_of_credit_card_numbers#Python https://en.wikipedia.org/wiki/Luhn_algorithm """

    r = [int(ch) for ch in str(n)][::-1]
    return (sum(r[0::2]) + sum(sum(divmod(d * 2, 10)) for d in r[1::2])) % 10 == 0