Atn Parsing

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  • 1. Parsing: Features & ATN & Prolog By [email_address]
  • 2. Feature System
    • CFG – inconvenient for capturing NL.
    • Most NL are often agreement restrictions between words and phrases.
    • For example:
      • *a men (a man) : a indicates single object while the noun men indicates a plural object
      • The noun phrase does not satisfy the Number Agreement restriction of English
    • Other forms of agreements:
      • Subject-verb
      • Gender agreement for pronouns
      • Restriction between the head of a phrase & the form of its complement
  • 3. Feature System cont.
    • To handle such phenomena:
      • Grammatical formalism is extended to allow constituent to have Features
      • Eg: define a feature NUMBER that may take a value of either s (singular) or p (plural).
  • 4. Augmented Transition Network (ATN)
    • RTN –> add features = ATN
    • The features are called Registers
    • Constituent structures are created by allowing each network to have a set of registers.
  • 5. ATN processes
    • When a network is pushed, a new set of registers is created
    • As the network is traversed, the registers are set to values by actions associated with each arc.
    • When the network is popped, the register are assembled to form a constituent structure, with the CAT slot being the network name
  • 6. Example ATN Grammar
    • Example Grammar 4.11 (pg 102, Allen) & 4.12
  • 7. Grammar & Logic Programming
    • Another popular method is to encode the rules of grammar into logic programming language such as Prolog.
    • Prolog uses the same search strategy: depth first top-down parsing algorithm
    • Just need to reformulate CFG as clauses in PROLOG
  • 8. Example
    • S -> NP VP
    • S(P1,P3):- np(P1,P2), vp(P2,P3).
      • Axiom: “There is an S between position 1 and position 3, if there is a position p2 such that there is an NP between p1 and p2 and a VP between p2 and p3”.
    • Add axioms listing the words in the sentence (‘John ate the cat’) by their position:
      • word(john,1,2).
      • word(ate,2,3).
      • word(the,3,4).
      • word(cat,4,5).
  • 9. Example cont.
    • The Lexicon is defined by a set of predicates:
      • isart(the).
      • isname(john).
      • isverb(ate).
      • isnoun(cat).
      • Ambiguous words would produce multiple assertions – one for each syntactic category to which they belong
  • 10. Example cont.
    • For each syntactic category, define a predicate that is true only if the word between the two specified positions is of that category, for example:
      • n(I,O):- word(Word,I,O), isnoun(Word).
      • art(I,O):- word(Word,I,O), isart(Word).
      • v(I,O):- word(Word,I,O), isverb(Word).
      • name(I,O):- word(Word,I,O), isname(Word).
  • 11. Prolog-based Parsing
    • Prove that “John ate the cat” is a legal sentence.
    • ie. Prove : S(1,5).
    • Refer the tracing example in page 74, Allen