A Hybrid Approach To The Automatic Planning Of Textual Structures

1. Introduction Over the last decade, the research goals in natural language generation have shi fred fi'om the generation of isolated sentences to the production of coherent multi-sentence paragraphs. Two major aspects of the generation process have been focused on: deciding 'what to say' (the strategic level) and deciding 'how to say it' (the tactical level). In 1985, McKeown designed one of the first systems to produce paragraphs using so-called sche-mata to describe conventional text structures in terms of patterns. Schemata are used to determine the content and order of the clauses in paragraphs (McKeown, 1985). However, these structures have a major limitation (Moore and Paris, 1988): schemata do not contain a description of tim intentional and rhetorical role that each part of the paragraph plays with respect to the whole paragraph. In 1988, How first employed RST (Pdmtorical Structure Theory) relations, which state the relationships between individual elements of a text, to control the construction of texts (How, 1988). In developing this RST-based method, Ho W has discovered that RST relations are a powerful tool for planning pa,'agraphs. They support reasoning about the intentions of writers and readers in a very natural way. Planning with rhetorical relations affords more flexibility than schcnmta. This method of planning paragraphs builds a tree structure that represents the internal organisation and rhetorical dependencies between clauses in a text. P, nt there is a cost: it is more difficult to assemble an RST paragraph tree from a set of independent relations than it is to instantiate and traverse a schema (Hovy, 1991). In 1992, Hovy et. al. described a new text planner (Ho Wet. al., 1992) that identifies the distinct types of knowledge necessary to generate coherent discourse in a text generation system. These knowledge resources are integrated under a planning process that draws from appropriate resources whatever knowledge is needed to construct a text. Though Itovy et. al. do not claim to have identified all the knowledge sources required to produce coherent discourse, their planner sets a trend for applying multi-knowledge resources for more complete and flexible phmning of text.