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L03 FS: Feature structures with CFG Features and CFG We have already seen that feature structures (FS) can be associated with words in the lexicon: + FS associated with 'child' in lexicon [ ROOT child ] [ CAT n ] [ AGRMNT [ NUMBER sg ] ] [ [ PERSON 3 ] ] + FS associated with 'a' in lexicon [ SPEC indef ] [ AGRMNT [ NUMBER sg ] ] [ [ PERSON 3 ] ] o Features can be put in context free grammar (CFG) - Associate FS with both terminals & non-terminals - Specify constraints on the CFG + Example CF rule 1 with constraint + NP -> det n + Constraint: FS(NP) = FS(det) U FS(n) - That is, NP consists of a det followed by an n, * provided, the FS of det and n unify. * resulting FS of NP is the result of unification + Resulting FS (of NP for 'a child') [ ROOT child ] [ CAT n ] [ SPEC indef ] [ AGRMNT [ NUMBER sg ] ] [ [ PERSON 3 ] ] o Continuing the example: + CF rule 3: VP -> v NP Constraints: FS(VP).act = FS(v).root FS(VP).obj = FS(NP) + CF rule 2: S -> NP VP Constraints: FS(S).agent = FS(NP) FS(S).patient = FS(VP).obj FS(S).act = FS(VP).act o Builds a semantic structure above for the sentence + 'a child eats the cake': [ CAT S ] [ AGENT [ ROOT child ] ] [ [ ... ] ] [ ] [ ACT [ ROOT eat ] ] [ [ ... ] ] [ ] [ OBJ [ ROOT cake ] ] [ [ ... ] ] -------------------------------------------- o Subj-verb agreement rule can also be expressed by means of feature structures: + CF rule 2: S -> NP VP Additional constraint: FS(NP).AGRMNT = FS(VP).AGRMNT - S can be rewritten to: NP followed by VP, provided: * Value of attribute AGRMNT in FS associated with NP, unifies with that in VP + CF rule 3: VP -> v NP Additional constraints: FS(VP).AGRMNT = FS(v).AGRMNT FS(VP).TENSE = FS(v).TENSE Note: VP gets its agreement features from v, and not NP o Example application of rules with input sentence: A child sees the toys. - FS with 'picks' and VP (constraint with rule 3): [TENSE prsnt ] [AGRMNT [NUMBER sg] ] - Constraint with rule 2 succeeds because values of AGRMNT feature are compatible...........................................NEXT PREVIOUS INDEX