这只适于一次性博弈下，来复盘各种情形找出最优。在多次博弈下，就不能每次总是不退让，而是首次退让，以后每次追随对手前一次所为。

The strategy is simply to cooperate on the first iteration of the game; after that, the player does what his or her opponent did on the previous move。it will not defect before its opponent does 。