length-normalise LM score

def norm(self):
        """
        length-normalise LM score
        """
        for (k, _) in self.entries.items():
            labelingLen = len(self.entries[k].labeling)
            self.entries[k].prText = self.entries[k].prText ** (1.0 / (labelingLen if labelingLen else 1.0))