Earlier quoted context omitted.
In the card game, the rule is that if a card has a vowel on one side, it must have an even number on the other side. Replace "the card has a vowel on one side" with "a person is drinking alcohol" and "it must have an even number on the other side" with "he or she must be over eighteen". Since a card with a vowel must have an even number on it, if a card has an odd number, one must check whether the other side has a v…
Sorry, but I still think that checking what a very old person is drinking is stupid, no matter how much notation you add to it.
Yes, that's the point. The bouncer doesn't need to check the old man, and the card player doesn't need to check the third card.
This is obvious when given the bar context, but less obvious when the same set of rules is taken out of any human context.