Sequencing as a Planning Task

I n t r o d u c t i o n The extent to which the components of a text or utterance succeed in carrying out their intended function depends in part on the sequence in which they are realized. For example, a critical aspect of understanding an explanation is to integrate the concepts and propositions in the explanation with existing knowledge. Sequencing decisions should a t tempt to facilitate this integration and otherwise enhance the intended functionality of the segments of the explanation. Superficially, the "sequential structure = of discourse is simply the order in which its elements are positioned in a linear medium. However, some of the ordering may be arbitrary. In a theoretical analysis, it is more useful to define the seq u e n t i a l s t r u c t u r e of discourse as a partial ordering that has specific justifications. This paper provides a collection of such justifications in the form of strategies for translating functionally significant relationships between discourse elements into palrwise ordering I constraints between those elements. We begin with a discussion of the nature of the se1 "Ordering" and "sequencing" are used interchangeably. quencing task and the advantages of explicit operators for this task. A number of strategies for the coherent ordering of an explanation are then presented, some of which were used in an earlier implemented system [Suthers 1993a], and many of which are justified by prior psycholinguistic research. Directions for further research are also discussed, including current efforts to extend the work to focus structure in general (i.e., subordination structure as well as sequential structure), and to address the handling of conflicts between strategies (e.g., between centering theory and McKeown's focus preferences). A p p r o a c h e s t o t h e S e q u e n c i n g T a s k Previous work in generation has handled sequencing decisions in a number of ways. Schematic approaches specify allowable orderings implicitly in terms of the transitions of finite state au tomata [McKeown 1985]. Nondeterminism in these au tomata has been addressed using focus preferences for selecting from a content pool, these preferences being embodied in a selection mechanism. Other approaches exploited the structure of domain knowledge with mechanisms for traversing data structures representing this knowledge [Paris & McKeown 1986, Sibun 1992]. Planning approaches initially utilized more local yet still schematic specifications of ordering, expressed as preconditions or optional satellites for plan operators [Cawsey 1989, Hovy 1988, Moore 1989]. More recently, partial order causal link (POCL) planning is being applied to discourse planning, with partial ordering derived in a principled manner from the relationships between preconditions and postconditions of plan steps [Young et gl. 1994]. Content selection processes provide some important constraints on sequential structure, most notably in the form of satisfaction-precedence relations derived from the preconditions and effects of discourse planning operators. However these processes underconstraln sequential structure. This is especially true below the granularity at which the operators interface with a domain knowledge source, because the latter is partially responsible for providing collections of related content that can' t be specified by domain-independent opera. tors. For example, a specification that some distinguish-