A Method for Disjunctive Constraint Satisfaction

A distinctive property of many current grammatical formalisms is their use of feature equal ity constraints to express a wide variety of grammatical dependencies Lexical Functional Grammar Head Driven Phrase Structure Grammar PATR FUG and the various forms of categorial uni cation grammar all require an analysis of a sentence to satisfy a collection of feature constraints in addition to a set of conditions on the arrange ment of words and phrases Conjunctions of equality constraints can be quickly solved by standard uni cation algorithms so they in themselves do not present a computational problem However the equality constraints derived for typical sentences are not merely con joined together in a form that uni cation algorithms can deal with directly Rather they are embedded as primitive elements in complex disjunctive formulas For some formalisms these disjunctions arise from explicit disjunction operators that the constraint language pro vides for e g LFG while for others disjunctive constraints are derived from the application of alternative phrase structure rules e g PATR In either case disjunctive speci cations help to simplify the statement of grammatical possibilities Alternatives expressed locally within individual rules and lexical entries can appeal to more general disjunctive processing mechanisms to resolve their global interactions