  In this paper we prove that the inequality introduced by Collins, Gisin,
Linden, Massar and Popescu is tight, or in other words, it is a facet of the
convex polytope generated by all local-realistic joint probabilities of d
outcomes. This means that this inequality is optimal. We also show that, for
correlation functions generalized to deal with three-outcome measurements, the
satisfyability of this inequality is a necessary and sufficient condition for
the existence of a local-realistic model accounting for them.
