Rock, Paper, Scissors, Lizard, Spock
Yep, rock, paper, scissors, (and now the extension of) lizard and Spock. Let Sheldon explain it:
Btw, for those who will start looking for a pen and paper to double check the logic (c'mon, you know you will :), here it is; each node has two incoming and two outgoing arrows:
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ref: Sam Kass