Before Herman Cain bows out of the presidential race to take an afternoon slot on Fox News, let us pause to consider seriously his famous 9-9-9 plan. Could a nine-percent national sales tax ever become law? Not very likely, argues Katherine Mangu-Ward of Reason in a Bloggingheads.tv diavlog with Benjy Sarlin of TalkingPointsMemo. Why not? Try asking Grandma what she thinks of the idea.