Order-Sorted Termination: The Unsorted Way

We consider the problem of proving termination of ordersorted rewrite systems. The dominating method for proving termination of order-sorted systems has been to simply ignore sort information, and use the techniques developed for unsorted rewriting. The problem with this approach is that many order-sorted rewrite systems terminate because of the structure of the set of sorts. In these cases the corresponding unsorted system would not terminate. In this paper we approach the problem of order-sorted termination by mapping the order-sorted rewrite system into an unsorted one such that termination of the latter implies termination of the former. By encoding sort information into the unsorted mapping, we are able to use general purpose termination orderings to prove termination of order-sorted rewrite systems whose termination depend on the sort hierarchy. We present a sequence of gradually stronger methods, and show that a previously published method is contained in ours as a special case.