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

CHAPTER 5. PROBABILISTIC ATTRIBUTE GRAMMARS 1115.3.3 Feature operatorsTwo new operators are necessary to perform generalizations involving feature equations. Featuremerging is the obvious step of merging two feature names, similar to the way nonterminals are merged.Consider the grammarS --> Det N V A NS.tr = N(1).f1S.lm = N(2).f2S.rel = V.hN --> circleN.f1 = ‘circle’--> squareN.f2 = ‘square’Now features f1, f2 are merged into a single feature f, denoted by fmerge(f1, f2) = f.S --> Det N V A NS.tr = N(1).fS.lm = N(2).fS.rel = V.hN --> circleN.f = ‘circle’--> squareN.f = ‘square’Feature equation counts are preserved in this step, and counts are added where equations merge as a result ofthe renaming of features.The other feature operation, feature attribution, postulates that a certain feature valueiassignedto a LHS nonterminal is actually due to a RHS nonterminal )fattrib , )g?is0&6Lg. 3and its feature )g£jg. The operation is writtenTo illustrate, consider rules as they could have been created by sample incorporationand subsequentmerging of lexical categories.S --> A CIRCLE IS BELOW A SQUARES.tr = ‘circle’S.lm = ‘square’S.rel = ‘below’S --> A SQUARE TOUCHES A TRIANGLES.tr = ‘circle’S.lm = ‘square’S.rel = ‘below’A --> aCIRCLE --> circleSQUARE --> squaretriangle --> triangleIS --> isBELOW --> belowTOUCHES --> touches3 The featurepis newly created, but an existing feature ofq can effectively be reused by subsequently merging it withp.