Proving Positive Almost Sure Termination Under Strategies

In last RTA, we introduced the notion of probabilistic rewrite systems and we gave some conditions entailing termination of those systems within a finite mean number of reduction steps. Termination was considered under arbitrary unrestricted policies. Policies correspond to strategies for non-probabilistic rewrite systems. This is often natural or more useful to restrict policies to a subclass. We introduce the notion of positive almost sure termination under strategies, and we provide sufficient criteria to prove termination of a given probabilitic rewrite system under strategies. This is illustrated with several examples.