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Elaborazione del linguaggio naturale

Elaborazione del linguaggio naturale. Fabio Massimo Zanzotto. Part six. Tree Adjoining Grammars. Our Aim. Lines of development Grammatical Representation Power: Build a formalism/model able to give the possibility of reducing the unnecessary interpretations

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Elaborazione del linguaggio naturale

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  1. Elaborazione del linguaggio naturale Fabio Massimo Zanzotto FMZ

  2. Part six Tree Adjoining Grammars FMZ

  3. Our Aim Lines of development Grammatical Representation Power: Build a formalism/model able to give the possibility of reducing the unnecessary interpretations Grammar Use: Build a formalism (and an associated algorithm) able to represent partial analysis FMZ

  4. Our Aim Lines of development Grammatical Representation Power: • CFG (context free grammars)  DCG • Feature Structures Grammar Use: • CYK • Chart and Early Algorithm FMZ

  5. Lesson learnt • Lexicon (i.e. words) is a very important piece of the Language and of the language model • Words carry meaning and govern the syntactic structure of sentences FMZ

  6. What we observed Toy Examples: ... La vecchia porta la sbarra ... ... Il vecchio porta la sbarra ... ... Flying planes can be dangerous ... ... Flying planes is dangerous ... FMZ

  7. Continuing the observation of the languages Some more toy examples: ... Il ragazzo mangia la mela con il coltello ... ... L’uomo guarda il monitor con gli occhi stralunati ... ... Le azioni della acme inc aumentano in tre settimane da 2 euro a 3 euro ... How many interpretations are possible for these sentences? FMZ

  8. Subcategorisation frames Necessary subcategorisation frames: ... Il ragazzo mangia la mela con il coltello ... ((NP) mangiare (NP) (PP(con))) ... L’uomo opera il paziente di appendicite ... ((NP) operare (NP) (PP(di))) ... Le azioni della acme inc aumentano in tre settimane da 2 euri a 3 euro ... ((NP) aumentare (PP(da)) (PP(a))) FMZ

  9. Modelling Subcat Frames in CFGs Target Frame: ((NP) mangiare (NP) (PP(con))) S  NP VP | NP VP(mangiare) NP  NP SBAR VP  VerbX NP | VerbX NP PP VerbX  Verb | Modal Verb VP(mangiare)  VerbX(mangiare) NP | VerbX (mangiare) NP PP(con) VerbX(mangiare)  Verb(mangiare) | Modal Verb(mangiare) NP  Art Noun | Art Adj Noun | Noun | Verb Noun | NP PP PP  Prep NP FMZ

  10. Observations Il ragazzo mangia la mela a mezzogiorno con il coltello How do we modify those(?): VP(mangiare)  VerbX(mangiare) NP | VerbX (mangiare) NP PP(con) VerbX(mangiare)  Verb(mangiare) | Modal Verb(mangiare) FMZ

  11. Summing up • We understood that subcategorisation can indicate preferred sentence readings • We want to build lexicalised rules, that is, rules governed by lexical elements (words) • We want to empower the grammar FMZ

  12. Idea!!! • Lexicalised rules may be partial tree! ((NP) aumentare (PP(da)) (PP(a))) S VP NP PP PP V NP IN IN NP aumentare da a FMZ

  13. Defining better our aim • We want a lexicalised grammar • each rule (partial tree) has to at least a lexical item • We want a grammar equivalent to the a given grammar • weak equivalence: equivalence in the language recognised • strong equivalence: equivalence in generated trees with respect to input sentences... remember that the structure define the “meaning” FMZ

  14. Operations in CFG in the derivation, no terminal symbols are substituted with rewriting rules headed by the same symbol may be understood as tree substitution no terminal symbols are substituted with trees headed by the same symbol is it sufficient to obtain the strong equivalence? FMZ

  15. Investigating strong equivalence Given the grammar and the sequence aaaa, one of the interpretations is: FMZ

  16. Let’s build the lexicalised grammar Given the tree collection the interpretation cannot be obtained! FMZ

  17. Another example Given the grammar it can be lexicalised as follows: strange rule!!! FMZ

  18. Another example The same grammar may be lexicalised also however, what about this FMZ

  19. Tree Adjoining Grammars FMZ

  20. What do we need? • A new operation!!! the Tree Adjoining operation FMZ

  21. Tree Adjoining Example FMZ

  22. Again the substitution • The well know operation FMZ

  23. Substitution example FMZ

  24. Tree Adjoining again Does it solve the problem of obtaining the strong equivalence? This is the solution to the example problem!!! FMZ

  25. Tree Adjoining again successive adjoining of (b4) FMZ

  26. More examples may give the interpretation: FMZ

  27. What is a grammar now? It is a collection of: • initial trees, represent the lexicon, e.g., • auxiliary trees, represent grammatical rules, e.g., FMZ

  28. Do you remember? copy-language structures: Pino, Gino e Rino sono rispettivamente fratello, zio e babbo di Nino may be read as: a1a2a3b1b2b3 Exercise: Find a model in TAG for this problem FMZ

  29. What we have done? • we have worked on the representation power of the grammar • we introduced: • lexicalised rules • the adjoining operation • where do we pay? • on the parsing algorithm? FMZ

  30. Question Have we resolved the problem of selecting between different readings (sentence interpretations)? FMZ

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