9. The method of claim 1 , further comprising: generating a new cluster node based on a given document; and generating new links between the new cluster and other cluster nodes assumed to be involved in generating the given document, or terminal nodes representing words in the given document, or both.
1. A method for constructing a model that generates text, the method comprising: in a computer system, performing the operations of: representing a concept as a cluster node; representing a word as a terminal node; assigning a weight to a link between two nodes; and training the model based on a set of documents, comprising: for each cluster node, computing a probabilistic cost of a corresponding concept existing in a document but not triggering any words.