11. The system of claim 10 , wherein the operations further comprise: determining, based on the probabilistic generative model, that the first node explains the second node; and 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 to generate the combined node comprises: based on determining that the first node explains the second node, 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 to generate the combined node.
11. The system of claim 10 , wherein the operations further comprise: determining, based on the probabilistic generative model, that the first node explains the second node; and 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 to generate the combined node comprises: based on determining that the first node explains the second node, 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 to generate the combined node. 12. The system of claim 11 , wherein the operations further comprise: receiving a set of training documents, wherein each training document contains a set of terms; applying the set of training documents to the probabilistic generative model to produce a new probabilistic generative model; and selecting the new probabilistic generative model as the probabilistic generative model.