Disjunctive Unification

This paper describes techniques for the representation and efficient unification of feature structures in the presence of disjunctive information. For the specification of feature structures we introduce feature terms which may contain sorts, variables, negation and named disjunction. Disjunction names allow for a compact representation and efficient management of multiple solutions even in cases where the differences between the solutions affect different parts of the structures. We present an algorithm for the unification of such structures in form of simplification rules for logical formulas which come with an open-world semantics. Departing from a single algorithm for the non-disjunctive case, we show how an algorithm for the disjunctive case can be derived using a general scheme originally proposed in [Maxwell & Kaplan, 1989]. We investigate an alternative representation, called context-unique feature descriptions, where a closer coupling between variables and disjunctive contexts allows major simplifications of the algorithm, which facilitate an efficient implementation. The methods proposed here are in no way limited to the field of natural language processing, but could be useful in other applications of logic programming, where the treatment of disjunctive information leads to efficiency problems within conventional approaches.