So as not to "cross the streams" here--which would be bad--my routine is a modest variation of a version of what most people seem to call "Positive/Negative". It is not the "Nightmare Coins" exactly, although there's a touch of that to it.
The routine appears on the COINvention discs, as part of a medley based on coin tricks where the spectators do more than just watch or hold things. It goes like this:
The magician claims he's holding two things, one is invisible, the other can't be seen. The invisible thing is a tiny coin purse containing three invisible coins. The thing they can't see is a prediction, and they can't see it because the magician has it in his closed hand. The prediction is given to a spectator to hold.
Of course, the coins are laid out and one is selected and flipped. The unchosen coins are returned to the tiny purse. The purse then appears at the magician's fingertips and it does contain the unselected coins. The prediction is shown to be a jumbo version of the chosen coin, proper side up.
I burden you with the details in order to get the credits straight--the basic routine is either Max Maven's or Larry Becker's. (I seem to recall the Becker routine as Bill does.) The idea of using a coin as a prediction, and the procedure that makes that possible is from my talented friend John George. You can find his routine on his videotape.
All I added was the tiny coin purse. It's (literally) no big thing, but it allows you to straddle the line between mentalism and magic, if that sort of anarchy appeals to you.