An abstract reduction system is a set of objects equipped with some binary “reduction” relations. As they have so little structure, abstract reduction systems can be viewed as abstractions of several kinds of rewriting such as string rewriting, term rewriting and graph rewriting. In the case of term rewriting, the objects model terms and reduction relations model (nondeterministic) computations. A desirable property in computing is that results of computations are unique (if they exist). In the case that whenever we have two “diverging” computations starting from the same term, a common result can be reached by “converging” computations(the so-called confluence or Church-Rosser property), uniqueness is guaranteed. ac