Since it is possible that different verbs may be associated with varying number of object arguments, we limit ourselves to verbs that take on at most two objects (or more specifically two noun phrase arguments) as a simplifying assumption: N k = {n 1 , n 2 } where n 2 can be NULL.
the rationale for using a HMM is that we can reuse all previous computation of the probabilities at each level to compute the required probabilities at the current level.