Fairbot does not play optimally on the PD\. $~$\\mathsf{Fairbot}$~$ does cooperate with another $~$\\mathsf {Fairbot}$~$, even if the code of the other agent is not exactly similar \(e\.g\. they can use different proof systems\)\. ? If $~$\\mathsf {Fairbot}$~$ is reasoning in a sound system such as first\-order arithmetic, then $~$\\mathsf {Fairbot}$~$ is also inexploitable; it never Cooperates when the opponent Defects\. However, $~$\\mathsf {Fairbot}$~$ cooperates with $~$\\mathsf {CooperateBot},$~$ the agent which simply always returns 'Cooperate'\. By the premises of the Prisoner's Dilemma, we ought to at least bother to Defect against a rock with the word "Cooperate" written on it\. $~$\\mathsf {Fairbot}$~$ fails to exploit $~$\\mathsf {CooperateBot},$~$ so $~$\\mathsf {Fairbot}$~$'s play is not optimal\.
use colon instead?