Allan: No, it's preferable to choose (D,C) if we assume that the other player bets on cooperation.

Which will happen only if the other player assumes that the first player bets on cooperation, which with your policy is incorrect. You can't bet on unstable model.

decide self.C; if other.D, decide self.D We're assuming, I think, that you don't get to know what the other guy does until after you've both committed (otherwise it's not the proper Prisoner's Dilemma). So you can't use if-then reasoning.

I can use reasoning, but not actual reaction on the facts, which are inaccessible. I debug my model of decision-making policies of both myself and other player, by requiring the outcome to be stable even if I assume that we both know which policy is used by another player (within a single model). Then I select the best stable model.