6. The method of claim 1 , further comprising: selecting N-best-scored child nodes from one or more levels of the hierarchy, wherein the N-best-scored child nodes have highest probabilities among child nodes of their level, wherein N is an integer; and determining the subset of the state variables according to the probabilities of the N-best scored child nodes.
6. The method of claim 1 , further comprising: selecting N-best-scored child nodes from one or more levels of the hierarchy, wherein the N-best-scored child nodes have highest probabilities among child nodes of their level, wherein N is an integer; and determining the subset of the state variables according to the probabilities of the N-best scored child nodes. 7. The method of claim 6 , wherein N varies among the levels of the hierarchy.