A Computational Theory of Prose Style for Natural Language Generation

In this paper we report on initial research we have conducted on a computational theory of prose style. Our theory speaks to the following major points: 1. Where in the generation process style is taken into account. 2. How a particular prose style is represented; what "stylistic rules" look like; 3. What modifications to a generation algor i thm are needed; what the deci~'on is that evaluates stylistic alternatives; 4. What elaborations to the normal description of surface structure are necessary to make it usable as a plan for the text and a referenco for these decicions; 5. What kinds of information decisions about style have access to. Our theory emerged out of design experiments we have made over the past year with our natural language generation system, the Zetalisp program MUMBLE. In the process we have extended MUMBLE through the addition of an additional process that now mediates between content planning and linguistic realization. This new process, which we call "attachment", provides the further si~,nificant benefit that text structure is no longer dictated by the structure of the message: the sequential order and dominance relationships of concepts in the memage no longer force one form onto the words and p h ~ in the text. Instead, rhetorical and intentional directives can be interpreted flexibly in the context of the ongoing discour~ and stylistic preferences. The text is built up through compos/tion under the direction of Linguistic organly/nS principles, rather than having to follow conceptual principles in Iockstep. We will begin by describing what we mean by prose style and then introducing the generation task that lead us to this theory, the reproduction of short encyclopedia articles on African tribes. We will then use that task to outline the parts of our theory and the operations of the attachment process. Finally we will compare our techniques to the related work of Davey, McKeown and Derr, and Gabriel, and consider some of the possible psychollnguistic hypotheses that it may lead to.