docs: def checksum(sin):
      """
      Determine validity of a Canadian Social Insurance Number.
      Validation is performed using a modified Luhn Algorithm.  To check
      the Every second digit of the SIN is doubled and the result is
      summed.  If the result is a multiple of ten, the Social Insurance
      Number is considered valid.
  
      https://en.wikipedia.org/wiki/Social_Insurance_Number
      """
  
      # Remove spaces and create a list of digits.
      checksumCollection = list(sin.replace(' ', ''))
      checksumCollection = [int(i) for i in checksumCollection]
  
      # Discard the last digit, we will be calculating it later.
      checksumCollection[-1] = 0
  
      # Iterate over the provided SIN and double every second digit.
      # In the case that doubling that digit results in a two-digit
      # number, then add the two digits together and keep that sum.
  
      for i in range(1, len(checksumCollection), 2):
          result = checksumCollection[i] * 2
          if result < 10:
              checksumCollection[i] = result
          else:
              checksumCollection[i] = result - 10 + 1
  
      # The appropriate checksum digit is the value that, when summed
      # with the first eight values, results in a value divisible by 10
  
      check_digit = 10 - (sum(checksumCollection) % 10)
      check_digit = (0 if check_digit == 10 else check_digit)
  
      return check_digit