We can restrict the effective size of X (2) by assigning zero probability to the vast majority of its elements, by constraining the hypothesis space to consider only cluster assignment functions q(x i |c i ) in which n 2 << V 2 of the elements have nonzero probability.
every word sequence in V d can be generated by the d = n segmentation, but every other segmentation is constrained to generate at most a subset of V d with nonzero probability.