HMM: avoid O(N) lookups. This makes HiddenMarkovModelTrainer.train_supervised 30x+ times faster.
HMM: avoid O(N) lookups. This makes HiddenMarkovModelTrainer.train_supervised 30x+ times faster.

Note: demo_pos() execution time is currently dominated by entropy calculations.

nltk/nltk,nltk/nltk,nltk/nltk