docs: def checksum(string):
      """
      Compute the Luhn checksum for the provided string of digits. Note this
      assumes the check digit is in place.
      """
      digits = list(map(int, string))
      odd_sum = sum(digits[-1::-2])
      even_sum = sum([sum(divmod(2 * d, 10)) for d in digits[-2::-2]])
      return (odd_sum + even_sum) % 10