  Okay, Roberts, Bartlett and Raussendorf recently introduced a new
cohomological approach to contextuality in measurement based quantum
computation. We give an abstract description of their obstruction and the
algebraic structure it exploits, using the sheaf theoretic framework of
Abramsky and Brandenburger. At this level of generality we contrast their
approach to the Cech cohomology obstruction of Abramsky, Mansfield and Barbosa
and give a direct proof that Cech cohomology is at least as powerful.
