Parsing Against Lexical Ambiguity

PARSING AGAINST LEXICAL AMBIGUITY Rob Milne Dept of Artificial Intelligence and School of Epistemics University of Edinburgh Edinburgh, Scotland EH8 9NW ABSTRACT Marcus' original deterministic parsing included almost no part-of-speech ambiguity. In this paper, the addition of part-of-speech ambiguity to a deterministic parser written in Prolog is described. To handle this ambiguity, it was necessary to add no special mechanisms to the parser. Instead the grammar rules were made to enforce agreement, and reject ungrammatical sentences. The resulting system is very effective and covers many examples of ambiguity. INTRODUCTION Most words can be more than one part of speech. For example, many words that can be a noun, can also be a verb, many -ing verbs can also act as adjectives, many prepositions can serve as particles, several modals can also be nouns, and some relative pronouns can also be determiners. In order to analyze a sentence, it is necessary to decide which part of speech a given word is in the sentence. Deciding which part of speech a word is during sentence processing shall be refered to as Lexical Ambiguity(LA). If a parser is to handle a wide range of English and ambiguity, it is necessary for it to handle this problem. STATE OF THE ART Marcus [1977] showed that a wide range of English grammar could be parsed determinsitca!ly, that is without every making a mistake and having to backtrack. But in Marcus' parser, almost every word was defined as only one part of speech. For example in his parser, "block" could only be a noun, making the following sentence unacceptable to the parser. [I] Block the road. With so little ambiguity, it is not surprising that Marcus's parser could work deterministically. For determinstic parsing to be a serious claim, it must be shown that it is possible to parse determinstically sentences which contain part-of-speech ambiguity. Is deterministic parsing still possible when part of speech ambiguity is included? The answer to this question can be thought of as the first major test for determinsitic parsing. If it is able to handle part-of-speech ambiguity easily, this will be a major reinforcement of the deterministic parsing strategy. If it cannnot handle LA, the theory will collapse. The first approach to LA for a deterministic parser was [Milne 78]. This work dealt solely with noun/verb ambiguity. When a noun/verb word was discovered, a special packet of rules was activated to decide which part-of-speech the word should be. For example, a typical rule stated that "to" followed by a noun/verb word meant that the noun/verb word was being used as a verb, and would disambiguate it as such. The rest of the grammar dealt with the disambiguated word. Although this approach was very effective, the rules were very special case, and many rules would be needed to handle all the possibilities. THE DEFAULT CASE I have implemented a deterministic parser in Prolog [Pereira 78] similar to Marcus' but extended it to allow words to be defined as multiple parts of speech. The …

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