How Much is worth to remember? A Taxonomy based on Petri Nets Unfoldings

The notion of unfolding plays a major role in the so called non sequential semantics of Petri nets, as well as in model checking of concurrent and distributed systems or in control theory. In literature various approaches to this notion have been proposed, where dependencies among events are represented either taking into account the whole history of the event (the so called individual token philosophy) or considering the whole history irrelevant (the so called collective token philosophy). In this paper we propose two unfoldings where the history is partially kept. These notions are based on unravelling a net rather than unfolding it. We compare them with the classical ones and we put all of them together in a coherent framework.