Clark, just a mail to remind you to code flow.zip. with merge :[1,Cooperate(),2] + [3,4] = [1,3,4,2] with zip, it should be [1,Cooperate(),2] + [3,4] = [(1,3)(2,4)] Hmm, By the way, Cooperate() isn't clear, even with the doc... :-( could you explain me that please ? thx Philippe