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After the iterated prisoner's dilemma tournament organized by prase two years ago, there was discussion of running tournaments for several variants, including one in which two players submit programs, each of which are given the source code of the other player's program, and outputs either “cooperate” or “defect”. However, as far as I know, no such tournament has been run until now.

Here's how it's going to work: Each player will submit a file containing a single Scheme lambda-function. The function should take one input. Your program will play exactly one round against each other program submitted (not including itself). In each round, two programs will be run, each given the source code of the other as input, and will be expected to return either of the symbols “C” or “D” (for "cooperate" and "defect", respectively). The programs will receive points based on the following payoff matrix:

$\begin{array}{cccc} & C & D & other\\ C & (2,\,2) & (0,\,3) & (0,\,2)\\ D & (3,\,0) & (1,\,1) & (1,\,0)\\ other & (2,\,0) & (0,\,1) & (0,\,0) \end{array}$

“Other” includes any result other than returning “C” or “D”, including failing to terminate, throwing an exception, and even returning the string “Cooperate”. Notice that “Other” results in a worst-of-both-worlds scenario where you get the same payoff as you would have if you cooperated, but the other player gets the same payoff as if you had defected. This is an attempt to ensure that no one ever has incentive for their program to fail to run properly, or to trick another program into doing so.

Your score is the sum of the number of points you earn in each round. The player with the highest score wins the tournament. Edit: There is a 0.5 bitcoin prize being offered for the winner. Thanks, VincentYu!

Details:
All submissions must be emailed to wardenPD@gmail.com by July 5, at noon PDT (Edit: that's 19:00 UTC). Your email should also say how you would like to be identified when I announce the tournament results.
Each program will be allowed to run for 10 seconds. If it has not returned either “C” or “D” by then, it will be stopped, and treated as returning “Other”. For consistency, I will have Scheme collect garbage right before each run.
One submission per person or team. No person may contribute to more than one entry. Edit: This also means no copying from each others' source code. Describing the behavior of your program to others is okay.
I will be running the submissions in Racket. You may be interested in how Racket handles time (especially the (current-milliseconds) function), threads (in particular, “thread”, “kill-thread”, “sleep”, and “thread-dead?”), and possibly randomness.
Don't try to open the file you wrote your program in (or any other file, for that matter). I'll add code to the file before running it, so if you want your program to use a copy of your source code, you will need to use a quine. Edit: No I/O of any sort.
Unless you tell me otherwise, I assume I have permission to publish your code after the contest.
You are encouraged to discuss strategies for achieving mutual cooperation in the comments thread.
I'm hoping to get as many entries as possible. If you know someone who might be interested in this, please tell them.
It's possible that I've said something stupid that I'll have to change or clarify, so you might want to come back to this page again occasionally to look for changes to the rules. Any edits will be bolded, and I'll try not to change anything too drastically, or make any edits late in the contest.

Here is an example of a correct entry, which cooperates with you if and only if you would cooperate with a program that always cooperates (actually, if and only if you would cooperate with one particular program that always cooperates):

(lambda (x)
(if (eq? ((eval x) '(lambda (y) 'C)) 'C)
'C
'D))