Proof that Raymond Smullyan is a genius: he named this idiosyncratic subsection of logic, and generated a bunch of examples (some printed in his The Riddle of Scheherazade). Here is a version of one of them: the sultan promises Scheherazade to truthfully answer her next yes or no question with a yes or a no, as a boon to a doomed woman, for he intends to have her executed in the morning . She asks something like, "Will you either answer no to this question or spare my life?" If he answers no, then he has claimed that he would not answer no and not spare her, so he has lied. Therefore, he must answer yes. However, since he did not answer no, his answer can only be true if he spares her life. Thus, merely by guaranteeing an honest answer, he has allowed her to coerce him into sparing her.

Because anything can be substituted for the sparing of Scheherazade's life, this basic idea has lots of applications. For example, in the original set of Jyhad, a trading card game now renamed to Vampire: The Eternal Struggle and based on White Wolf's Vampire: the Masquerade, there was a card called the Monacle of Clarity, which allowed a player to compel another player to truthfully answer a yes or no question. This led to the possibility of some truly absurd consequences in the game, such as a player being forced to concede merely because of one use of the monacle. Worse still, however, the question was never limited to the game, and so one could ask about (and thus, compel actions regarding)things like sexual favors, destroying the world, etc.--this resulted in people breaking the rules by failing to obey the text on the card (shocker).

It's also a nice way to deal with questions like, "If you could ask one yes-or-no question of God, what would it be?"

"Will you either answer no to this question, or let me into Heaven?" Heck, I could get greedier than that, but I don't REALLY want to run the whole show, anyway. Interestingly, there are ways to be coercive without the restriction to yes-or-no questions, but they're sort of complex (enough so that I forget exactly what they are, and would need to refer to my book again to find them, which is currently impossible because one of my students, showing good taste, stole it last year).