jurafsky&martin_3rdEd_17 (1).pdf

Other kinds of evidence for constituency come from

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Other kinds of evidence for constituency come from what are called preposed or Preposed postposed constructions. For example, the prepositional phrase on September sev- Postposed enteenth can be placed in a number of different locations in the following examples, including at the beginning (preposed) or at the end (postposed): On September seventeenth , I’d like to fly from Atlanta to Denver I’d like to fly on September seventeenth from Atlanta to Denver I’d like to fly from Atlanta to Denver on September seventeenth But again, while the entire phrase can be placed differently, the individual words making up the phrase cannot be *On September , I’d like to fly seventeenth from Atlanta to Denver *On I’d like to fly September seventeenth from Atlanta to Denver *I’d like to fly on September from Atlanta to Denver seventeenth See Radford (1988) for further examples of groups of words behaving as a single constituent. 11.2 Context-Free Grammars The most widely used formal system for modeling constituent structure in English and other natural languages is the Context-Free Grammar , or CFG . Context- CFG free grammars are also called Phrase-Structure Grammars , and the formalism is equivalent to Backus-Naur Form , or BNF . The idea of basing a grammar on
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170 C HAPTER 11 F ORMAL G RAMMARS OF E NGLISH constituent structure dates back to the psychologist Wilhelm Wundt (1900) but was not formalized until Chomsky (1956) and, independently, Backus (1959) . A context-free grammar consists of a set of rules or productions , each of which Rules expresses the ways that symbols of the language can be grouped and ordered to- gether, and a lexicon of words and symbols. For example, the following productions Lexicon express that an NP (or noun phrase ) can be composed of either a ProperNoun or NP a determiner ( Det ) followed by a Nominal ; a Nominal in turn can consist of one or more Noun s. NP ! Det Nominal NP ! ProperNoun Nominal ! Noun | Nominal Noun Context-free rules can be hierarchically embedded, so we can combine the pre- vious rules with others, like the following, that express facts about the lexicon: Det ! a Det ! the Noun ! flight The symbols that are used in a CFG are divided into two classes. The symbols that correspond to words in the language (“the”, “nightclub”) are called terminal Terminal symbols; the lexicon is the set of rules that introduce these terminal symbols. The symbols that express abstractions over these terminals are called non-terminals . In Non-terminal each context-free rule, the item to the right of the arrow ( ! ) is an ordered list of one or more terminals and non-terminals; to the left of the arrow is a single non-terminal symbol expressing some cluster or generalization. Notice that in the lexicon, the non-terminal associated with each word is its lexical category, or part-of-speech, which we defined in Chapter 10.
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