Representing Paraphrases Using Synchronous TAGs

Representing Paraphrases Using Synchronous TAGs Mark Dras Microsoft Research Institute, Macquarie University NSW Australia 2109 markd~mpce, mq. edu. au Abstract This paper looks at representing para- phrases using the formalism of Syn- chronous TAGs; it looks particularly at comparisons with machine translation and the modifications it is necessary to make to Synchronous TAGs for paraphrasing. A more detailed version is in Dras (1997a). 1 Introduction The context of the paraphrasing in this work is that of Reluctant Paraphrase (Dras, 1997b). In this framework, a paraphrase is a tool for modify- ing a text to fit a set of constraints like length or lexical density. As such, generally applicable para- phrases are appropriate, so syntactic paraphrases-- paraphrases that can be represented in terms of a mapping between syntax trees describing each of the paraphrase alternatives--have been chosen for their general applicability. Three examples are: (1) a. The salesman made an attempt to wear Steven down. b. The salesman attempted to wear Steven down. (2) a. The compere who put the contestant to the lie detector gained the cheers of the audience. b. The compere put the contestant to the lie detector test. He gained the cheers of the audience. (3) a. The smile broke his composure. b. His composure was broken by the smile. A possible approach for representing paraphrases is that of Chandrasekar et al (1996) in the context of text simplification. This involves a fairly straightfor- ward representation, as the focus is on paraphrases which simplify sentences by breaking them apart. However, for purposes other than sentence simplifi- cation, where paraphrases like (1) are used, a more complex representation is needed. A paraphrase representation can be thought of as comprising two parts--a representation for each of the source and target texts, and a representation for mapping between them. Tree Adjoining Gram- mars (TAGs) cover the first part: as a formalism for describing the syntactic aspects of text, they have a number of desirable features. The proper- ties of the formalism are well established (Joshi et al, 1975), and the research has also led to the de- velopment of a large standard grammar (XTAG Re- search Group, 1995), and a parser XTAG (Doran et al, 1994). Mapping between source and target texts is achieved by an extension to the TAG formalism known as Synchronous TAG, introduced by Shieber and Schabes (1990). Synchronous TAGs (STAGs) comprise a pair of trees plus links between nodes of the trees. The original paper of Shieber and Schabes proposed using STAGs to map from a syntactic to a semantic representation, while another paper by Abeill@ (1990) proposed their use in machine trans- lation. The use in machine translation is quite close to the use proposed here, hence the comparison in the following section; instead of mapping between possibly different trees in different languages, there is a mapping between trees in the same language with very different syntactic properties. 2 Paraphrasing with STAGs Abeill~ notes that the STAG formalism allows an explicit semantic representation to be …

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