A system of seven coherence relations for hierarchically organizing event concepts in text (nexus)
Richard Alterman · 1982
A theory of event concept coherence is developed. It is shown that the pieces of event description which appear in a body of text can be gathered together and hierarchically organized using a dictionary of event/state concepts. This theory has been implemented in a computer system, NEXUS. The representations it produces are constructed in terms of sevel coherence relations. The dictionary it uses was compiled from an analysis of ten folktales. The seven coherence relations used are class/subclass, sequence/subsequence, coordinate, antecedent, precedent, consequent, and sequel. Class/subclass is a taxonomic relation. Sequence/subsequence and coordinate are two kinds of whole/part relations. The other four relations are temporal; antecedent and precedent concepts come before an event, consequent and sequel concepts come after an event. Associations between concepts in the dictionary are also organized in terms of the seven coherence relations. Associated with the concepts in the dictionary are default values for its case related arguments. Relationships between concepts in the dictionary are refined by attaching to each relationship in the dictionary a set of constraints on matching case arguments. NEXUS uses the default values and the constraints to control the representation building process. The flexibility of the representation scheme is demonstrated by applying it to several diverse examples of narrative text in the literature, including script and plan based stories. The feasability of NEXUS is shown by applying it to eight samples of text and discussing in detail the results. The utility of the hierarchical representation produced by NEXUS is validated by experiments in question answering and summarizing.