I don't like too much this implementation. It has multiple calls to .index() which from my understand will iterate over the deck lists until it finds the element. So worst case scenario, it iterates over the same list 4 times. In this case the deck list is short so performance is no problem but it could become a problem with longer lists of cards.

In doing the abbreviations kata which shortens words longer than 4 letters to first and last letter with a number for letters between, what standard is used? There is nothing in Wikipedia or other places describing the process. The requirements listed for the kata are insufficient to complete it. Unknown error is not feedback.