the theorem can be improved to ${\mathbb C}^3$ at the cost of using a more complicated example, as in the original Kochen-Specker proof.

> [[Q: ]{}]{}This is too bad that there is such a brutal limitation on observation spaces. And, since the theorem speaks about modeling in an observation frame, rather than observation space, negative probabilities are irrelevant.
>
> [[A: ]{}]{}It is not completely obvious to us at this point that negative probabilities are irrelevant. We can weaken conditions M3 and M4 by requiring that the equalities hold only up to an error of probability zero. The resulting weaker models would be defined not in observation frames but in observation spaces because the weaker notion involves probabilities.
>
> If we stick to nonnegative probabilities, this doesn’t buy us anything. But once negative probabilities enter the picture, errors of probability zero become more complicated. They may involve cancellation between outcomes of positive probability and outcomes of negative probability.
>
> [[Q: ]{}]{}I have another question. It seems that the proof of the limitative Theorem \[thm:limit\] crucially uses the existence of points in observation frames. Did you consider working more abstractly with just observable events and coobservable sets of events?
