Generate a random United States Taxpayer Identification Number of the specified type.

        If no type is specified, a US SSN is returned.

def ssn(self, taxpayer_identification_number_type=SSN_TYPE):
        """ Generate a random United States Taxpayer Identification Number of the specified type.

        If no type is specified, a US SSN is returned.
        """

        if taxpayer_identification_number_type == self.ITIN_TYPE:
            return self.itin()
        elif taxpayer_identification_number_type == self.EIN_TYPE:
            return self.ein()
        elif taxpayer_identification_number_type == self.SSN_TYPE:

            # Certain numbers are invalid for United States Social Security
            # Numbers. The area (first 3 digits) cannot be 666 or 900-999.
            # The group number (middle digits) cannot be 00. The serial
            # (last 4 digits) cannot be 0000.

            area = self.random_int(min=1, max=899)
            if area == 666:
                area += 1
            group = self.random_int(1, 99)
            serial = self.random_int(1, 9999)

            ssn = "{0:03d}-{1:02d}-{2:04d}".format(area, group, serial)
            return ssn

        else:
            raise ValueError("taxpayer_identification_number_type must be one of 'SSN', 'EIN', or 'ITIN'.")