Representing Paraphrases Using Synchronous TAGs

This paper looks at representing paraphrases using the formalism of Synchronous 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 I n t r o d u c t i o n The context of the paraphrasing in this work is that of Reluctant Paraphrase (Dras, 1997b). In this framework, a paraphrase is a tool for modifying a text to fit a set of constraints like length or lexical density. As such, generally applicable paraphrases 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 at tempt 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 straightforward representation, as the focus is on paraphrases which simplify sentences by breaking them apart. However, for purposes other than sentence simplification, where paraphrases like (1) are used, a more complex representation is needed. A paraphrase representation can be thought of as comprising two par t s -a representation for each of the source and target texts, and a representation for mapping between them. Tree Adjoining Grammars (TAGs) cover the first part: as a formalism for describing the syntactic aspects of text, they have a number of desirable features. The properties of the formalism are well established (Joshi et al, 1975), and the research has also led to the development of a large standard grammar (XTAG Research 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 translation. 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 P a r a p h r a s i n g w i t h S T A G s Abeill~ notes that the STAG formalism allows an explicit semantic representation to be avoided, mapping from syntax to syntax directly. This fits well with the syntactic paraphrases described in this paper; but it does not, as Abeill@ also notes, preclude semantic-based mappings, with Shieber and Schabes constructing syntax-to-semantics mappings as the first demonstration of STAGs. Similarly, more semantically-based paraphrases are possible through an indirect application of STAGs to a semantic representation, and then back to the syntax. One major difference between use in MT and paraphrase is in lexicalisation. The sorts of mappings that Abeill~ deals with are lexically idiosyncratic: the English sentences Kim likes Dale and Kim misses Dale, while syntactically parallel and semantically fairly dose, are translated to different