Returns the credit card type of a specified credit card number.
card_num - the credit card number to be validated.
CREDITCARDTYPE only determines the following credit card types :
For all other (mathematically) valid credit cards, CREDITCARDTYPE returns an answer of "Unkown".
The calculation of this function is based on the author’s limited knowledge in the topic concerned. This function is only a demonstration of programming techniques. It is not expected to be used in any business situations for the validation of credit card numbers.
refer to the “LIMITATION OF LIABILITY” paragraph
of the Excel Power Expander License Agreement.