docs: 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