The theory of Petri nets is a part of computer science whose importance
is increasingly acknowledged. Many papers and anthologies, whose subject
matter is net theory and its applications, have appeared to date. There
exist at least seven introductory textbooks on the theory. The present
monograph augments this literature by offering a mathematical treatment
of one of the central aspects of net theory: the modelling of concur-
rency by partially ordered sets. Occurrence nets - which are special
nets as well as special partial orders - are proposed by net theory for
this purpose. We study both the general properties of occurrence nets
and their use in describing the concurrent behaviour of systems.
Occurrence nets may be contrasted with a more language-oriented approach
to the modelling of concurrency known as arbitrary interleaving. We will
dis- cuss some connections between these' two approaches. Other
approaches based on partially ordered sets - such as the theory of
traces, the theory of event structures and the theory of semi words -
are not considered in this book, in spite of the strong links between
them and net theory.