1. A computer-implemented method performed by a data processing apparatus, the method comprising: receiving a node of a Hidden Markov Model, wherein the node is a label transition node; receiving a frame, wherein the frame is a predicted segmentation point; and pruning the node from a possible nodes list with label transition node pruning, wherein label transition node pruning comprises: scoring the node at the frame to obtain a score, and pruning the node when the score is greater than the sum of a best score at the frame and a beam threshold minus a penalty term.