# Unification Grammars

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Unification Grammars. Allen s Chapter 4 J&M s Chapter 11. Unification of Feature Structures. Key concept - extension relationship between two FSs: F2 extends F1 if every feature-value pair in F1 is also in F2 - PowerPoint PPT Presentation

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• *Unification GrammarsAllens Chapter 4J&Ms Chapter 11

600.465 Intro to NLP J. Eisner

• *Unification of Feature StructuresKey concept - extension relationship between two FSs:F2 extends F1 if every feature-value pair in F1 is also in F2Two FSs F1 and F2 unify if there exists a FS F that is an extension of both of themThe Most General Unifier is the minimal FS F that extends both The Unification operation allows easy expression of grammatical relationships among constituent feature structures

• *Unification of Feature Structures

F2 extends F1:F1 = (CAT v) F2 = (CAT v ROOT cry)F1 = (AGR {3s 3p}) F2 = (AGR 3s)

F3 is MGU of F1 and F2:F1 = (CAT v ROOT cry), F2 = (CAT v VFORM pres), F3 = (CAT v ROOT cry VFORM pres)

F1 and F2 do not unify:F1 = (CAT v AGR 3s) F2 = (CAT v AGR 3p)

• *Unification-based GrammarsGrammar rules can be completely specified using unification Example:X0 --> X1 X2 CAT X0 = SCAT X1 = NPCAT X2 = VPAGR0 =AGR1 = AGR2VFORM0 = VFORM2 If a feature (such as CAT) is always specified, it can be associated with the non-terminal of a CFG rule Examples:S NP VPAGR0= AGR1 = AGR2VFORM0 = VFORM2NP ART N AGR0 = AGR1 = AGR2

• *Sample Grammar (Abbreviated Form)

• *Unification Grammars

• *Predicative PhrasesVP (V ROOT be) (NP PRED +)He is a studentVP (V ROOT be) (PP PRED +)He is in the houseVP (V ROOT be) (ADJP PRED +)He is happyX0 X1 X2 CAT0 =VPCAT1 = VCAT2 = {NP PP ADJP}ROOT1 = bePRED2 = +

• *More Information in Lexiconput : (CAT V SUBCAT (FIRST (CAT NP) SECOND (CAT PP LOC +)))

VP V X2 X32 = FIRSTSUBCAT13=SECONDSUBCAT1

want : (CAT V SUBCAT (FIRST (CAT NP) SECOND (CAT VP VFORM inf)))