The model is defined in terms of state transition probabilities P(c | c), i.e., the probability of transitioning from a state labeled c to a state labeled c ; and state observation probabilities P(w | c), i.e., the probability of emitting word w from a particular class c. Since the classes are predicting an ordering, we include hard constraints on class transitions.
we forbid a transition from a class closer to the head noun to one farther away.