When someone calls Mondo, you may answer with a black or white stone, or secretly abstain by putting neither stone in your fist. When you abstain, you get nothing either way. If you answer correctly, you get one guessing stone. If you answer incorrectly, you lose all your guessing stones.
I haven't had a chance to try this yet. If anyone else feels like giving it a whirl, please post back to the list and tell us how it went!