def of number
      raise_on_type_mismatch number
      digits = convert_number_to_digits(number)
      digits.reduce(0){ |check, digit| QUASIGROUP[check][digit] }
    end
def of number
      raise_on_type_mismatch number
      digits = convert_number_to_digits(number)
      digits.reduce(0){ |check, digit| QUASIGROUP[check][digit] }
    end
Calculates Damm checksum

 @example
   Sjekksum::Damm.of(572) #=> 4

 @param  number [Integer, String] number for which the checksum should be calculated

 @return [Integer] calculated checksum