A Meta-Level Grammar: Redefining Synchronous TAG for Translation and Paraphrase

A Meta-Level Grammar: Redefining Synchronous TAG for Translation and Paraphrase Mark Dras Microsoft Research Institute Department of Computer Science Macquarie University, Australia markd@±cs, mq. edu. au Abstract In applications such as translation and paraphrase, operations are carried out on grammars at the meta level. This pa- per shows how a meta-grammar, defining structure at the meta level, is useful in the case of such operations; in particu- lar, how it solves problems in the current definition of Synchronous TAG (Shieber, 1994) caused by ignoring such structure in mapping between grammars, for appli- cations such as translation. Moreover, es- sential properties of the formalism remain unchanged. 1 Introduction A grammar is, among other things, a device by which it is possible to express structure in a set of entities; a grammar formalism, the con- straints on how a grammar is allowed to ex- press this. Once a grammar has been used to express structural relationships, in many ap- plications there are operations which act at a 'meta level' on the structures expressed by the grammar: for example, lifting rules on a depen- dency grammar to achieve pseudo-projectivity (Kahane et al, 1998), and mapping between synchronised Tree Adjoining Grammars (TAGs) (Shieber and Schabes, 1990; Shieber 1994) as in machine translation or syntax-to-semantics transfer. At this meta level, however, the oper- ations do not themselves exploit any structure. This paper explores how, in the TAG case, us- ing a meta-level grammar to define meta-level structure resolves the flaws in the ability of Syn- chronous TAG (S-TAG) to be a representation for applications such as machine translation or paraphrase. This paper is set out as follows. It describes the expressivity problems of S-TAG as noted in Shieber (1994), and shows how these occur also in syntactic paraphrasing. It then demon- strates, illustrated by the relative structural complexity which occurs at the meta level in syntactic paraphrase, how a meta-level gram- mar resolves the representational problems; and it further shows that this has no effect on the generative capacity of S-TAG. 2 S-TAG and Machine Translation Synchronous TAG, the mapping between two Tree Adjoining Grammars, was first proposed by Shieber and Schabes (1990). An applica- tion proposed concurrently with the definition of S-TAG was that of machine translation, map- ping between English and French (Abeill~ et al, 1990); work continues in the area, for example using S-TAG for English-Korean machine trans- lation in a practical system (Palmer et al, 1998). In mapping between, say, English and French, there is a lexicalised TAG for each language (see XTAG, 1995, for an overview of such a gram- mar). Under the definition of TAG, a grammar contains elementary trees, rather than flat rules, which combine together via the operations of substitution and adjunction (composition oper- ations) to form composite structures--derived trees--which will ultimately provide structural representations for an input string if this string is grammatical. An overview of TAGs is given in Joshi and Schabes (1996). The characteristics of TAGs make them better suited to describing natural language …

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