AB S'll'RAC'I' in this paper we present a new dimension to paraphrasing text in which characteristics of the original text motivate strategies for effective pacaphrasing. Our system combines two existing robust components: the IRIJS-.II natural language underst~mding system and the SPOKESMAN generation system. We describe the architectur(: of the system and enhancements made to these components to facilitate paraphrasing. We particularly look at how levels of representation in these two systems are used by specialists in the paraphraser which define potential problems and paraphrasing strategies. Finally, we look at the role of paraphrasing in a cooperative dialog system. We will focus here on paraphrasing in the coutext of natural language interfaces and particularly on how multiple int::rpretations introduced by various kinds of ambiguity can be contrasted in paraphrases using both sentence structure and highlighting and folmating the text itself. While tecimieally paraphrasing is simply the task of restating the meaning of a text in a different form, it is crucial to consider the purpose of the paraphrase in order to motivate particular strategies for changinl: the text. If the t)oint of the paraphrase is to clarify the original texi, its in a natural language (NL) interface to a database (DB) or cx~rt system application, then disambiguating the que W and choosing more precise lcxical items (perhaps closer to the structure of the actual Dt3, expert system, o1' other underlying application) are essential strategies. If the point is to summarize information, then strategies for evaluating the relative importance of the information presenlcd in the text m'e necessary. If the point is merely to re:;tate the text ~t_.f~r~l lil2 than the original, perhaps merely to exercise the system, then one must use strategies which consider what structures and lexical items were actually found by the parser. Oar motivation for work on strategies for effective paraphrasing comes front the recent availablility of NI, interfaces as commercial products. As the underlying systems that a NL interface must interact with increase in number and sophistication, the range of NL interactions will increase as well. Paraphrasers developed in the past (e.g. McKeown's Co-op and BBN's Parlance'rMNL Interface) were all limited in that each used only a single strategy for paraphrasing regardless of what problems may have been present in the original query. (We diseussthese systems in detail in Section 6.) Our approach is to develop a variety of strategies which may be employed in different situations. We introduce a new dimension to paraphrasing text in which characteristics of the original text plus the overall context (inch~ding the goal of the system) mofiwtte strategies for effective paraphrasing. Our focus here will be on paraphrasing anlbiguous queries in an interactive dialog system, whc~e conlrasting nmltiple interpretations is essential. In order to ground our discussion, we first look briefly at a range of ambiguity types. We then provide an overview of the architecture and description of the two major components: the IRUS-II'rM mlderstanding system and the Spokesman generation system. We look closely at the aspects of these systems that we augmented t0r the paraphrasiug task and provide a detailed example of how the system appreciates multiple interpretations and uses that information to govern decision making in generation. Next we discuss the role of paraphrasing in a cooperative dialog system, and in the final section we conta'ast our approach with other work in paraphrasing. I We would like to Ihank Lance Ramshaw tot' his invaluable help in understanding die inner workings of RUS and suggestions of where it could be augmented for out' purposes, and Dawn MacLaughlin for her implementation of Pal~rot, the init~d versio, of our paraphraser. We would also like tx) thank Ralph Weisclmdel, D~mafis Ayuso, and David iglcDonald for their helpful comments of d~afl.s of this paper and Lya Bates tot early inspirations. Ambiguity is one of the more difficult problems to detect and correct. In this section we look at three kinds of ambiguity: lexical, structural and contextual, and discuss potential strategies a paraphraser might use to eliminate the ambiguity. 1) LEXICAL AMBIGUITIES ale introduced when a lexical item can refer to more than one thing. In the following example "Manhattan" can refer to either the borough of New York City or the ship: The paraphraser must appreciate the ambiguity of that noun phrase, decide how to disambiguate it, and decide how much of the context to include in the paraphrase. One strategey would be to repeat the entire query, disambiguating the noun phrase by using the type and name of the object: However, if the query is long, the result could be quite cumbersome. A different strategy, highlighting and formatting the text to contrast the differences, can serve to direct the user's attention to the part that is ambiguous: 2) STRUCTURAL AMBIGUITIES are caused when there are multiple parses for a sentence. Conjunction is a typical source of structural ambiguity. Modifiers of conjoined NPs may distribute over each NP or modify only the closest NP. Consider, for example, the followfi~g query: This query has only one interpretation in which the premodifier "forested" modifies only the noun "hills". In contrast, the following query has two interpretations: In one interpretation, the premodifier "CI" may apply only to the noun "carrier"; in the other, "CI" applies to both "carriers" and "frigates". Each interpretation requires a different paraphrase strategy. In the case where the premodifier distributes, the ambiguity may be eliminated by repeating the modifier: Disl)lay the C1 carr&rs and C1 frigates. When it does not distribute, there are three potential slxategies: --changing the order of the conjuncts: Display the frigates and C1 carr&rs. --hatrodueing explicit quantifiers: Display the C1 carriers and all the frigates. --moving premodifiers to postmodifiers: Display the carriers which are C1 arm the frigates. 3) CONTEXTUAL AMBIGUITIES are introduced when the query is underspecified for the underlying system it is working with. For example if the context includes a map and the possibility of natural language or table output, the query Which carriers are C1? could mean either list or display.
Meteer and Shaked REF ) discussed paraphrasing strategies for displaying several ambiguities of inputs by users in an interactive dialog system.
Strategies For Effective Paraphrasing
International Conference On Computational Linguistics
Computer Science