Mirror Paradox

Counter target spell. You can pay {x} and use the countered spell effect, where X is the total cost of the countered spell. After you use this effect, the owner of the countered spell can pay its cost and use the effect of its spell. If your opponent do this, you can again pay {x} and continue the cycle until someone cancel the effect. When this happen, the player who cancel the effect discard the top five cards of his library.