"Not a credit card" is returned on obviously invalid data values.
As of 0.30, cardtype() will accept a partial card masked with "x", "X', ".",
-"*" or "_". Only the first 2-6 digits and the lenth are significant;
+"*" or "_". Only the first 2-6 digits and the length are significant;
whitespace and dashes are removed. To recognize just Visa, MasterCard and
Amex, you only need the first two digits; to recognize almost all cards
except some Switch cards, you need the first four digits, and to recognize