  We submitted two kinds of strategies to the iterated prisoner's dilemma (IPD)
competitions organized by Graham Kendall, Paul Darwen and Xin Yao in 2004 and
2005. Our strategies performed exceedingly well in both years. One type is an
intelligent and optimistic enhanced version of the well known TitForTat
strategy which we named OmegaTitForTat. It recognizes common behaviour patterns
and detects and recovers from repairable mutual defect deadlock situations,
otherwise behaving much like TitForTat. The second type consists of a set of
strategies working together as a team. These group strategies have one
distinguished individual Godfather strategy that plays OmegaTitForTat against
non-members while heavily profiting from the behaviour of the other members of
his group, the Hitman. The Hitman willingly let themselves being abused by
their Godfather while themselves lowering the scores of all other players as
much as possible, thus further maximizing the performance of their Godfather in
relation to other participants. The study of collusion in the simplified
framework of the iterated prisoner's dilemma allows us to draw parallels to
many common aspects of reality both in Nature as well as Human Society, and
therefore further extends the scope of the iterated prisoner's dilemma as a
metaphor for the study of cooperative behaviour in a new and natural direction.
We further provide evidence that it will be unavoidable that such group
strategies will dominate all future iterated prisoner's dilemma competitions as
they can be stealthy camouflaged as non-group strategies with arbitrary
subtlety. Moreover, we show that the general problem of recognizing stealth
colluding strategies is undecidable in the theoretical sense.
