its eigenvalues. By observing whether $v$ is in $V$ we know whether $E$ occurred. So $E$ should be observable in $(\Omega,{\ensuremath{\mathrm{CO}}})$.
>
> The same justification applies to M2.
>
> [[Q: ]{}]{}Actually M2 implies M1.
>
> [[A: ]{}]{}Almost. M1 contains the requirement that $\mu(E)$ is defined which is used implicitly in M2.

\[thm:limit\] There is a finite set ${\ensuremath{\mathcal S}}$ of Hermitian operators on ${\mathbb C}^4$ that admits no model in any observation frame.

Consider the 18-vector, 9-basis example of Cabello et al. [@Cabello] given in color on the Wikipedia page for “Kochen-Specker Theorem” [@W-KS]. For each of the 9 bases $B$ there, invent an operator $S_B$ (Hermitian, with 4 distinct eigenvalues) whose eigenspaces are exactly the 4 vectors in $B$ (and their scalar multiples). Let ${\ensuremath{\mathcal S}}$ be the set of these 9 $S_B$’s, and suppose, toward a contradiction, that $\mu$ were a model of ${\ensuremath{\mathcal S}}$ in $(\Omega,{\ensuremath{\mathrm{CO}}})$.

Let $B$ be any one of the nine bases, and consider the sums $E$ of eigenspaces of $S_B$, i.e., the “coordinate subspaces” of ${\mathbb C}^4$ with respect to the basis $B$. They constitute a Boolean algebra ${\mathrm{BA}}(B)$ and, by the requirements for a model, their $\mu$-images are defined 