Petri net processes for zero-safe nets

Composition of modules to larger units is a technique frequently used during the software development life cycle. it is mostly used in a "bottom Lip" fashion, suggested by the principles of object orientation, where the composition of simple objects to a complex one plays a central role. Composition in Petri nets has been studied in the form of place and transition fusion. Zero-Safe Nets represent a special approach, which allows the use of more complex synchronisation structures, so-called transactions. The definition of transactions is based on interleaving semantics, i.e. on firing-sequences. Problems arise, since the definition is not closed with respect to the permutation of actions. This paper presents a partial order concurrency semantics for zero-safe nets based on Petri net processes. Using these semantics, a characterisation of such transactions closed with respect to permutation of concurrent actions becomes possible.