A survey of basic net models and modular net classes

The paper surveys those net classes which can be called to some extent `modular' and the basic net models used as a framework in their definition. In particular the first part introduces condition event systems, elementary net systems place transition systems and 1-safe systems, adhering as much as possible to the original definitions, with the few modifications consolidated in the literature. It discusses and compares the basic net models by considering how each one of them deals with some fundamental properties of a net model, such as simplicity, pureness, backward and forward reachability, liveness, contact-freeness. The second part surveys the main classes of modular nets defined in the literature, showing that most of them share some basic features, since they typically refer to a common idea of building the overall net by composing the nets modelling its sequential components by means of state machines. The differences are considered from the perspective of the specific goal and field of interest of the various authors, and the technical apparatus associated with the various net classes is briefly referred to and illustrated by examples