Grammar Rewriting

We present a term rewriting procedure based on congruence closure that can be used with arbitrary equational theories. This procedure is motivated by the pragmatic need to prove equations in equational theories where confluence can not be achieved. The procedure uses context free grammars to represent equivalence classes of terms. The procedure rewrites grammars rather than terms and uses congruence closure to maintain certain congruence properties of the grammar. Grammars provide concise representations of large term sets. Infinite term sets can be represented with finite grammars and exponentially large term sets can be represented with linear sized grammars. Although the procedure is primarily intended for use in nonconfluent theories, it also provides a new kind of confluence that can be used to give canonical rewriting systems for theories that are difficult to handle in other ways. For example, under grammar rewriting there is a finite canonical rewrite system for idempotent semigroups, a theory which has been shown not to have any finite canonical system under traditional notions of rewriting. Copyright @) Massachusetts Institute of Technology, 1991 This report describes research done at the Artificial Intelligence Laboratory of the Massachusetts Institute of Technology. Support for the work described in this paper was provided in part by Misubishi Electric Research Laboratories, Inc. Support for the laboratory's artificial intelligence research is provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014-4-K-0124. 93-01677 3llllllll ,93 128 014