3. The method of claim 1 , wherein training the model based on the documents further comprises: for a given link, producing a weight function for each document; and optimizing a product of all the weight functions for the given link.
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.