That struck me as well, but thinking about it a bit more, I could see there being a set of correct decisions in trading, which could be taken as assumptions. I'm not sure such a set exists, but after playing a good deal of Catan, I think they might. In a game where everyone knows what they're doing, the trades are very predictable, and it's pretty clear when someone has made a dumb trade. It seems like there are still too many unknowns to consider it "solvable", but it seems less open ended than poker, for instance.