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Unification Grammars

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

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Unification Grammars

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  1. Unification Grammars Allen’s Chapter 4 J&M’s Chapter 11

  2. 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 • Two FSs F1 and F2 unify if there exists a FS F that is an extension of both of them • The Most General Unifier is the minimal FS F that extends both • The Unification operation allows easy expression of grammatical relationships among constituent feature structures

  3. 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)

  4. Unification-based Grammars • Grammar rules can be completely specified using unification • Example: X0 --> X1 X2 CAT X0 = S CAT X1 = NP CAT X2 = VP AGR0 =AGR1 = AGR2 VFORM0 = 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 VP AGR0= AGR1 = AGR2 VFORM0 = VFORM2 NP  ART N AGR0 = AGR1 = AGR2

  5. Sample Grammar (Abbreviated Form)

  6. Unification Grammars

  7. Predicative Phrases • VP  (V ROOT be) (NP PRED +) He is a student • VP  (V ROOT be) (PP PRED +) He is in the house • VP  (V ROOT be) (ADJP PRED +) He is happy • X0  X1 X2 CAT0 =VP CAT1 = V CAT2 = {NP PP ADJP} ROOT1 = be PRED2 = +

  8. More Information in Lexicon put : (CAT V SUBCAT (FIRST (CAT NP) SECOND (CAT PP LOC +))) VP  V X2 X3 2 = FIRSTSUBCAT1 3=SECONDSUBCAT1 want : (CAT V SUBCAT (FIRST (CAT NP) SECOND (CAT VP VFORM inf)))

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