29. An apparatus for generating text, the apparatus comprising: a modeling mechanism in a computer system configured to: represent a concept as a cluster node; represent a word as a terminal node; assign a weight to a link from a first node to a second node, wherein the weight indicates a probability of the second node triggering additional nodes when the first node triggers the second node; a model-training mechanism in the computer system configured to train the model based on a set of documents, wherein the training comprises: for each cluster node, computing a probabilistic cost of a corresponding concept existing in a document but not triggering any words.
29. An apparatus for generating text, the apparatus comprising: a modeling mechanism in a computer system configured to: represent a concept as a cluster node; represent a word as a terminal node; assign a weight to a link from a first node to a second node, wherein the weight indicates a probability of the second node triggering additional nodes when the first node triggers the second node; a model-training mechanism in the computer system configured to train the model based on a set of documents, wherein the training comprises: for each cluster node, computing a probabilistic cost of a corresponding concept existing in a document but not triggering any words. 30. The apparatus of claim 29 , wherein the weight assigned to a link from a first node and a second node indicates a probability of the second node triggering additional nodes when the first node triggers the second node.