Planning Coherent Multisentential Text

Though most text generators are capable of simply stringing together more than one sentence, they cannot determine which order will ensure a coherent paragraph. A paragraph is coherent when the information in successive sentences follows some pattern of inference or of knowledge with which the hearer is familiar. To signal such inferences, speakers usually use relations that llnk successive sentences in fixed ways. A set of 20 relations that span most of what people usually say in English is proposed in the Rhetorical Structure Theory of Mann and Thompson. This paper describes the formalization of these relations and their use in a prototype text planner that structures input elements into coherent paragraphs. 1 T h e P r o b l e m o f C o h e r e n c e The example texts in this paper are generated by Penman, a systemic grammar-based generator with larger coverage than probably any other existing text generator. Penman was developed at ISI (see [Mann & Matthiessen 831, [Mann 831, [Matthiessen 84]). The input to Penman is produced by PEA (Programming Enhancement Advisor; see [Moore 87]), a program that inspects a user's LISP program and suggests enhancements. PEA is being developed to interact with the user in order to answer his or her questions about the suggested enhancements. Its theoretical focus is the production of explanations over extended interactions in ways that are superior to the simple goal-tree traversal of systems such as TYRESIAS ([Davis 76]) and MYCIN ([Shortliffe 76]). Supported by DARPA contract MDAg03 81 C0~5. In answer to the question how does the system enhance a program~, the following text (not generated by Penman) is not satisfactory: (a). The system performs the enhancement. Before *hat, the system resolves conficts. First, the system asks the user to tell Jt the characteristic of the program to be enhanced. The system app//es transformations to the program. / t confrms the enhancement with the user. It scans the program in order to find opportunities to apply transfarmations to the program. . . . because you have to work too hard to make sense of it. In contrast, using the same propositions (now rearranged and linked with appropriate connectives), paragraph (b) (generated by Penman) is far easier to understand: (b). The system as/ca ~he user to tell it the characteristic of the program to be enhanced. Then the system applies transformations to the program. In particular, the system scans the program in order to ~nd opportunities to apply transformations to the program. Then the system resolves contlicts. It con~rms the enhancement with the user. Fina//y, it performs the enhancement. Clearly, you do not get coherent text simply by stringing together sentences, even if they are related note especially the underlined text in (b) and its corresponding three propositions in (a). The goal of this paper is to describe a method of planning paragraphs to be coherent while avoiding unintended spurious effects that result from the juxtaposition of unrelated pieces of text.