Termination of a Set of Rules Modulo a Set of Equations

The problem of termination of a set R of rules modulo a set E of equations, called E-termination problem, arises when trying to complete the set of rules in order to get a Church-Rosser property for the rules modulo the equations. We first show here that termination of the rewriting relation and E-termination are the same whenever the used rewriting relation is E-commuting, a property inspired from Peterson and Stickel’s E-compatibility property. More precisely, their results can be obtained by requiring termination of the rewriting relation instead of E-termination if E-commutation is used instead of E-compatibility. When the rewriting relation is not E-commuting, we show how to reduce E-termination for the starting set of rules to classical termination of the rewriting relation of an extended set of rules that has the E-commutation property. This set can be classicaly constructed by computing critical pairs or extended pairs between rules and equations, according to the used rewriting relation. In addition we show that different orderings can be used for the starting set of rules and the added critical or extended pairs. Interesting issues for further research are also discussed.