Tit-for-tat is a ZD strategy which is "fair" in the sense of not gaining advantage over the other player. However, the ZD space also contains strategies that, in the case of two players, can allow one player to unilaterally set the other player's score or alternatively, force an evolutionary player to achieve a payoff some percentage lower than his own. The extorted player could defect but would thereby hurt himself by getting a lower payoff. Thus, extortion solutions turn the iterated prisoner's dilemma into a sort of ultimatum game. Specifically, "X" is able to choose a strategy for which formula_22, unilaterally setting formula_23 to a specific value within a particular range of values, independent of "Y" 's strategy, offering an opportunity for "X" to "extort" player "Y" (and vice versa). (It turns out that if "X" tries to set formula_24 to a particular value, the range of possibilities is much smaller, only consisting of complete cooperation or complete defection.)