Using Tree Adjoining Grammars Systemic Framework in the

In this paper we investigate the incorporation of Tree Adjoining Grammars (TAG) into the systemic framework. We show that while systemic grammars have many desirable characteristics as a generation paradigm, they appear to have problems in generating certain kinds of sentences (e.g., those containing discontinuity or long-distance dependencies). We argue that these problems can be overcome with an appropriate choice of structural units of realization. We show that TAG provides appropriate units of structural realization because they localize all dependencies and allow the realization of two independent subpieces to be interspersed with each other. We go on to show how TAG can be incorporated without affecting the basic tenants of systemic grammar. Finally, we indicate how the incorporation of TAG yields several benefits to the systemic framework. I n t r o d u c t i o n As pointed out by many researchers (e.g., [Davey 1978; Mann 1983; Matthiessen & Kasper 1985; Patten 1988; Bateman & Paris 1989]), systemic linguistics offers many advantages to a sentence generation component of a text generation system. Perhaps the strongest asset of systemics is its view of the generation process as a goal directed enterprise. Its emphasis is on function rather than form [Halliday 1985; Fawcett 1980; Hudson 1971], where the functional distinctions that are required in the grammar manifest themselves in the eventual output form. While systemic linguists have remained agnostic with respect to certain processing decisions, a computer implementation of a systemic grammar requires that explicit decisions be made concerning realization operators and the structures available for manipulation at each point in the processing. The explicit decisions that were made in previous implementations of systemic *This work is supported in part by Grant #H133ES0015 from the National Institute on Disability and Rehabilitation Research. Support was also provided by the Nemours Foundation. grammar (e.g., [Mann 1983; Mann & Matthiessen 1985; Matthiessen & Kasper 1985]) have proven to be problematic in some respects. In particular, the current implementations have difficulty in generating certain sentences which exhibit discontinuities or long distance dependencies. To date, these can only be handled in a limited fashion, and the solutions provided are not very satisfying. We argue that Tree Adjoining Grammar (TAG) provides a structural unit that is precisely appropriate for the implementation of a systemic grammar for the generation task. Moreover, we believe our use of TAG for this purpose is completely consistent with the systemic paradigm and helps to overcome the above difficulties. In this paper we first introduce the notion of a systemic grammar and the processing paradigm it espouses. We indicate problems with current implementations of this paradigm. Next, we introduce the notion of lexicalized Tree Adjoining Grammars, emphasizing their structural domains of locality, and justify that the basic structures of TAG are appropriate structures to be used in an implementation of a systemic grammar. Following this we indicate how a tree adjoining grammar can be used as the basis for an implementation of systemic grammar indicating the differences between the approach of current implementations of systemic grammar and that which would result from the incorporation of TAG. Finally, we indicate potential gains resulting from the incorporation of TAGs and the scope of current work. G e n e r a t i n g in S y s t e m i c P a r a d i g m :