8. The method of claim 1 , wherein obtaining the probabilistic generative model further comprises: obtaining a probabilistic generative model whose nodes (I) each correspond to a different cluster of terms in the probabilistic generative model, and (II) include (i) a first node that descends from one or more parent nodes, and (ii) one or more child nodes that descend from a second node.
8. The method of claim 1 , wherein obtaining the probabilistic generative model further comprises: obtaining a probabilistic generative model whose nodes (I) each correspond to a different cluster of terms in the probabilistic generative model, and (II) include (i) a first node that descends from one or more parent nodes, and (ii) one or more child nodes that descend from a second node. 9. The method of claim 8 , wherein determining to merge the first node that descends from the one or more parent nodes of the first node with the second node from which the one or more child nodes of the second node descend comprises: determining to merge the first node that corresponds to a first cluster of terms in the probabilistic generative model with the second node corresponds to a second cluster of terms in the probabilistic generative model to generate the combined node that corresponds to the first and second clusters of terms in the probabilistic generative model.