Proof Orders for Decreasing Diagrams

We present and compare some well-founded proof orders for decreasing diagrams. These proof orders order a conversion above another conversion if the latter is obtained by filling any peak in the former by a (locally) decreasing diagram. Therefore each such proof order entails the decreasing diagrams technique for proving confluence. The proof orders differ with respect to monotonicity and complexity. Our results are developed in the setting of involutive monoids. We extend these results to obtain a decreasing diagrams technique for confluence modulo

Compositional Confluence Criteria

We show how confluence criteria based on decreasing diagrams are generalized to ones composable with other criteria. For demonstration of the method, the confluence criteria of orthogonality, rule labeling, and critical pair systems for term rewriting are recast into composable forms. We also show how such a criterion can be used for a reduction method that removes rewrite rules unnecessary for confluence analysis. In addition to them, we prove that Toyama's parallel closedness result based on parallel critical pairs subsumes his almost parallel closedness theorem

Confluence in UnTyped Higher-Order Theories by means of Critical Pairs

User-defined higher-order rewrite rules are becoming a standard in proof assistants based on intuitionistic type theory. This raises the question of proving that they preserve the properties of beta-reductions for the corresponding type systems. We develop here techniques that reduce confluence proofs to the checking of various forms of critical pairs for higher-order rewrite rules extending beta-reduction on pure lambda-terms. The present paper concentrates on the case where rewrite rules are left-linear and critical pairs can be joined without using beta-rewrite steps. The other two cases will be addressed in forthcoming papers

From Diagrammatic Confluence to Modularity

International audienceThis paper builds on a fundamental notion of rewriting theory that characterizes confluence of a (binary) rewriting relation, Klop's cofinal derivations. Cofinal derivations were used by van Oostrom to obtain another characterization of confluence of a rewriting relation via the existence of decreasing diagrams for all local peaks. In this paper, we show that cofinal derivations can be used to give a new, concise proof of Toyama's celebrated modularity theorem and its recent extensions to rewriting modulo in the case of strongly-coherent systems, an assumption discussed in depth here. This is done by generalizing cofinal derivations to cofinal streams, allowing us in turn to generalize van Oostrom's result to the modulo case