jurafsky&martin_3rdEd_17 (1).pdf

Swissair to london satisfies the requirements for the

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SwissAir to London satisfies the requirements for the < F > operator, yielding the following derivation for our original example ( 11.11 ). flew IcelandAir to Geneva and SwissAir to London ( VP / PP ) / NP NP PP / NP NP CONJ NP PP / NP NP < T > < T > ( VP / PP ) \ (( VP / PP ) / NP ) PP ( VP / PP ) \ (( VP / PP ) / NP ) PP < T < T VP \ ( VP / PP ) VP \ ( VP / PP ) < < VP \ (( VP / PP ) / NP ) VP \ (( VP / PP ) / NP ) < F > VP \ (( VP / PP ) / NP ) < VP Finally, let’s examine how these advanced operators can be used to handle long- distance dependencies (also referred to as syntactic movement or extraction). As mentioned in Section 11.3.1 , long-distance dependencies arise from many English constructions including wh-questions, relative clauses, and topicalization. What these constructions have in common is a constituent that appears somewhere dis- tant from its usual, or expected, location. Consider the following relative clause as an example. the flight that United diverted Here, divert is a transitive verb that expects two NP arguments, a subject NP to its left and a direct object NP to its right; its category is therefore ( S \ NP ) / NP . However, in this example the direct object the flight has been “moved” to the beginning of the clause, while the subject United remains in its normal position. What is needed is a way to incorporate the subject argument, while dealing with the fact that the flight is not in its expected location. The following derivation accomplishes this, again through the combined use of type raising and function composition. the flight that United diverted NP / N N ( NP \ NP ) / ( S / NP ) NP ( S \ NP ) / NP > > T NP S / ( S \ NP ) > B S / NP > NP \ NP < NP As we saw with our earlier examples, the first step of this derivation is type raising United to the category S / ( S \ NP ) allowing it to combine with diverted via forward composition. The result of this composition is S / NP which preserves the fact that we are still looking for an NP to fill the missing direct object. The second critical piece is the lexical category assigned to the word that : ( NP \ NP ) / ( S / NP ). This function seeks a verb phrase missing an argument to its right, and transforms it into an NP seeking a missing element to its left, precisely where we find the flight .
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11.7 S UMMARY 193 CCGBank As with phrase-structure approaches, treebanks play an important role in CCG- based approaches to parsing. CCGBank (Hockenmaier and Steedman, 2007) is the largest and most widely used CCG treebank. It was created by automatically trans- lating phrase-structure trees from the Penn Treebank via a rule-based approach. The method produced successful translations of over 99% of the trees in the Penn Tree- bank resulting in 48,934 sentences paired with CCG derivations. It also provides a lexicon of 44,000 words with over 1200 categories. Chapter 13 will discuss how these resources can be used to train CCG parsers. 11.7 Summary This chapter has introduced a number of fundamental concepts in syntax through the use of context-free grammars .
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