“…A Petri net [1, 6,23] is an abstract, formal model of systems with interacting, concurrent or parallel components. Petri nets have been successfully used in modelling, validation and analysis of systems in which it is possible for some events to occur concurrently but there are constraints on the concurrence, precedence, or frequency of these occurrences [6,9,11,14,15,22]. Petri nets, however, are not complete enough for the study of performance issues since no assumption is made on the duration of systems activities.…”