The dissertation of Andreas Stolcke is approved: University of ...

CHAPTER 5. PROBABILISTIC ATTRIBUTE GRAMMARS 110Notice that the features cannot be attached to the lexical productions created, since that would implyknowing the semantic contributions of each word. Hence the features are first attached to the sentence as awhole, corresponding to rote learning of sentence-meaning associations.5.3.2 Nonterminal merging and chunkingThe nonterminal merging and chunking operators introduced in Chapter 4 can be applied to PAGrules with a few modifications. Merging in the CFG part of productions operates as usual, by renamingnonterminals and possibly merging productions. Feature equations are also affected by renaming nonterminals:Whenever two or more feature equations become identical as the result of nonterminal merging, theirassociated counts are added as usual.By way of example, suppose the we merge nonterminals N1 and N2, starting with productionsNP --> Det N1 (2)NP.f = N1.f1 (2)NP --> Det N2 (2)NP.f = N2.f2 (2)The result of merge(N1, N2) = N isNP --> Det N (4)NP.f = N.f1 (2)NP.f = N.f2 (2)Notice that the feature names f1 and f2 are not automatically merged; that will be accomplished by a separateoperator.Nonterminal chunking operates as before, but may require the creation of new ad hoc featuresattached to the newly created nonterminal in order to preserve any features found on the chunked nonterminals.This too is most easily seen by example:S --> Det N V A NS.tr = N(1).f (2)S.lm = N(2).f (2)S.rel = V.hWhen applying chunk(ANP:N) = NP the f feature must be preserved by creating a corresponding feature onS --> NP V NPS.tr = NP(1).g (2)S.lm = NP(2).g (2)S.rel = V.hNP --> Det NNP.g = N.f (4)