Payorian cooperation is easy with Kripke frames

The context is MIRI's twist on Axelrod's Prisoner's Dilemma tournament. Axelrod's competitors were programs, facing each other in an iterated Prisoner's Dilemma. MIRI's tournament is a one-shot Prisoner's Dilemma, but the programs get to read their opponent's code. Or, rather, a description of the behavior of the code in Gödel-Löb provability logic, which turns out to be enough to determine their behavior in the setup.