3. The method of claim 2 in which the nodes are coupled together by weighted links, so that if an incoming link from a node that is activated causes a cluster node to activate with a probability proportional to the weight of the incoming link, an outgoing link from the cluster node to another node causes the other node to fire with a probability proportionate to the weight of the outgoing link, otherwise the other node is not activated.
2. The method of claim 1 in which the observable variables are represented as terminal nodes, and the hidden variables are represented as cluster nodes.