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Showing papers on "Petri net published in 2003"


Book ChapterDOI
26 Jun 2003
TL;DR: The acronyms in this domain are tried to demystify, the state-of-the-art technology is described, and it is argued that BPM could benefit from formal methods/languages.
Abstract: Business Process Management (BPM) includes methods, techniques, and tools to support the design, enactment, management, and analysis of operational business processes. It can be considered as an extension of classical Workflow Management (WFM) systems and approaches. Although the practical relevance of BPM is undisputed, a clear definition of BPM and related acronyms such as BAM, BPA, and STP are missing. Moreover, a clear scientific foundation is missing. In this paper, we try to demystify the acronyms in this domain, describe the state-of-the-art technology, and argue that BPM could benefit from formal methods/languages (cf. Petri nets, process algebras, etc.).

1,480 citations


Journal ArticleDOI
01 Nov 2003
TL;DR: This paper introduces the concept of workflow mining and presents a common format for workflow logs, and discusses the most challenging problems and present some of the workflow mining approaches available today.
Abstract: Many of today's information systems are driven by explicit process models. Workflow management systems, but also ERP, CRM, SCM, and B2B, are configured on the basis of a workflow model specifying the order in which tasks need to be executed. Creating a workflow design is a complicated time-consuming process and typically there are discrepancies between the actual workflow processes and the processes as perceived by the management. To support the design of workflows, we propose the use of workflow mining. Starting point for workflow mining is a so-called "workflow log" containing information about the workflow process as it is actually being executed. In this paper, we introduce the concept of workflow mining and present a common format for workflow logs. Then we discuss the most challenging problems and present some of the workflow mining approaches available today.

1,168 citations


Book ChapterDOI
23 Jun 2003
TL;DR: The design principles, the basic concepts, and the underlying XML technology of PNML are discussed to stimulate discussion on and contributions to a standard Petri net interchange format.
Abstract: The Petri Net Markup Language (PNML) is an XML-based interchange format for Petri nets. In order to support different versions of Petri nets and, in particular, future versions of Petri nets, PNML allows the definition of Petri net types. Due to this flexibility, PNML is a starting point for a standard interchange format for Petri nets. This paper discusses the design principles, the basic concepts, and the underlying XML technology of PNML. The main purpose of this paper is to disseminate the ideas of PNML and to stimulate discussion on and contributions to a standard Petri net interchange format.

391 citations


Book ChapterDOI
23 Jun 2003
TL;DR: CPN Tools is a tool for editing, simulating and analysing Coloured Petri Nets, which is based on advanced interaction techniques, such as toolglasses, marking menus, and bi-manual interaction.
Abstract: CPN Tools is a tool for editing, simulating and analysing Coloured Petri Nets. The GUI is based on advanced interaction techniques, such as toolglasses, marking menus, and bi-manual interaction. Feedback facilities provide contextual error messages and indicate dependency relationships between net elements. The tool features incremental syntax checking and code generation which take place while a net is being constructed. A fast simulator efficiently handles both untimed and timed nets. Full and partial state spaces can be generated and analysed, and a standard state space report contains information such as boundedness properties and liveness properties. The functionality of the simulation engine and state space facilities are similar to the corresponding components in Design/CPN, which is a widespread tool for Coloured Petri Nets.

360 citations


Journal ArticleDOI
19 Feb 2003
TL;DR: This paper addresses the forbidden state problem of Petri nets with liveness requirement and uncontrollable transitions with the proposed approach computes a maximally permissive PN controller, whenever such a controller exists.
Abstract: This paper addresses the forbidden state problem of Petri nets (PN) with liveness requirement and uncontrollable transitions. The proposed approach computes a maximally permissive PN controller, whenever such a controller exists. The first step, based on a Ramadge-Wonham-like reasoning (1989), determines the legal and live maximal behavior the controlled PN should have. In the second step, the theory of regions is used to design control places to add to the original model to realize the desired behavior. Furthermore, necessary and sufficient conditions for the existence of control places realizing the maximum permissive control are given. A parameterized manufacturing application of significant state space is used to show the efficiency of the proposed approach.

333 citations


Journal ArticleDOI
TL;DR: A so-called true concurrency approach, in which no global state and no global time is available, is followed, which uses only local states in combination with a partial order model of time.
Abstract: In this paper, we consider the diagnosis of asynchronous discrete event systems. We follow a so-called true concurrency approach, in which no global state and no global time is available. Instead, we use only local states in combination with a partial order model of time. Our basic mathematical tool is that of net unfoldings originating from the Petri net research area. This study was motivated by the problem of event correlation in telecommunications network management.

297 citations


Book ChapterDOI
01 Jan 2003
TL;DR: The Petri Net Markup Language (PNML) is an XML-based interchange format for Petri nets that supports any version of Petri net since new PetriNet types can be defined by so-called Petrinet Type Definitions (PNTD).
Abstract: The Petri Net Markup Language (PNML) is an XML-based interchange format for Petri nets. PNML supports any version of Petri net since new Petri net types can be defined by so-called Petri Net Type Definitions (PNTD).

286 citations


Journal Article
TL;DR: Genomic Object Net as mentioned in this paper is a software tool for representing and simulating biopathways, which has been developed by employing the architecture of Hybrid Functional Petri Net (HFPN).
Abstract: The following two matters should be resolved in order for biosimulation tools to be accepted by users in biology/medicine: (1) remove issues which are irrelevant to biological importance, and (2) allow users to represent biopathways intuitively and understand/manage easily the details of representation and simulation mechanism From these criteria, we firstly define a novel notion of Petri net called Hybrid Functional Petri Net (HFPN) Then, we introduce a software tool, Genomic Object Net, for representing and simulating biopathways, which we have developed by employing the architecture of HFPN In order to show the usefulness of Genomic Object Net for representing and simulating biopathways, we show two HFPN representations of gene regulation mechanisms of Drosophila melanogaster (fruit fly) circadian rhythm and apoptosis induced by Fas ligand The simulation results of these biopathways are also correlated with biological observations The software is available to academic users from http://wwwGenomicObjectNet/

252 citations


Journal ArticleDOI
TL;DR: The goal of this paper is to establish a formal foundation for control-flow aspects of workflow specification languages, that assists in understanding fundamental properties of such languages, in particular their expressive power.
Abstract: Although workflow management emerged as a research area well over a decade ago, little consensus has been reached as to what should be essential ingredients of a workflow specification language. As a result, the market is flooded with workflow management systems, based on different paradigms and using a large variety of concepts. The goal of this paper is to establish a formal foundation for control-flow aspects of workflow specification languages, that assists in understanding fundamental properties of such languages, in particular their expressive power. Workflow languages can be fully characterized in terms of the evaluation strategy they use, the concepts they support, and the syntactic restrictions they impose. A number of results pertaining to this classification will be proven. This should not only aid those developing workflow specifications in practice, but also those developing new workflow engines.

238 citations


Book ChapterDOI
TL;DR: This tutorial introduces models, systems, and standards for the design, analysis, and enactment of workflow processes using Petri nets as a formal basis and exposes some of the typical semantic problems restricting the application of BPM/WFM technology.
Abstract: Over the last decade there has been a shift from “data-aware” information systems to “process-aware” information systems. To support business processes an enterprise information system needs to be aware of these processes and their organizational context. Business Process Management (BPM) includes methods, techniques, and tools to support the design, enactment, management, and analysis of such operational business processes. BPM can be considered as an extension of classical Workflow Management (WFM) systems and approaches. This tutorial introduces models, systems, and standards for the design, analysis, and enactment of workflow processes. Petri nets are used for the modeling and analysis of workflows. Using Petri nets as a formal basis, contemporary systems, languages, and standards for BPM and WFM are discussed. Although it is clear that Petri nets can serve as a solid foundation for BPM/WFM technology, in reality systems, languages, and standards are developed in an ad-hoc fashion. To illustrate this XPDL, the “Lingua Franca” proposed by the Workflow Management Coalition (WfMC), is analyzed using a set of 20 basic workflow patterns. This analysis exposes some of the typical semantic problems restricting the application of BPM/WFM technology.

234 citations


Journal ArticleDOI
TL;DR: This paper presents a meta-modelling scheme that automates the very labor-intensive and therefore time-heavy and therefore expensive process of manually winding down and restarting workflows.
Abstract: Workflow Modeling.- Workflow Design.- Performance Evaluation of Workflows.- Resource Allocation in Workflows.- Heuristic Workflow Redesign.- System and Practical Experience.- Conclusion.

BookDOI
01 Jan 2003
TL;DR: This talk presents the personal view of the current status of highlevel Petri nets, in particular Coloured Petri Nets, and the main challenges that need to be addressed.
Abstract: This talk presents my personal view of the current status of highlevel Petri Nets, in particular Coloured Petri Nets. What have we achieved over the last 25 years and what are the main challenges that need to be addressed in order to make high-level Petri nets more useful, e.g., to the software industry? What kind of language extensions and tool support do we need to make Petri net modelling, simulation and analysis accessible to the knowledgeable engineer? I will also briefly discuss the main areas which I believe are the most obvious candidates for further theoretical development.

Journal ArticleDOI
01 May 2003
TL;DR: A fuzzy reasoning algorithm is proposed to perform fuzzy reasoning automatically in the FRPN model to represent a fuzzy production rule-based system and allows one to exploit the maximum parallel reasoning potential embedded in the model.
Abstract: This paper presents a fuzzy reasoning Petri net (FRPN) model to represent a fuzzy production rule-based system. The issues of how to represent and reason about rules containing negative literals are addressed in the proposed PN model. The execution rules based on the model are defined formally using the operators in max-algebra. Then, a fuzzy reasoning algorithm is proposed to perform fuzzy reasoning automatically. The algorithm is consistent with the matrix equation expression method in the traditional PNs and allows one to exploit the maximum parallel reasoning potential embedded in the model. The legitimacy and feasibility of the proposed approach are proved and validated through a turbine fault diagnosis expert system.

Book ChapterDOI
TL;DR: The starting point in this paper is the α-algorithm for which it is proved under which conditions and process constructs the algorithm works, and a classification is given of the process constructs that are difficult to handle for this type of algorithms.
Abstract: Current workflow management systems require the explicit design of the workflows that express the business process of an organization. This process design is very time consuming and error prone. Considerable work has been done to develop heuristics to mine event-data logs to produce a process model that can support the workflow design process. However, all the existing heuristic-based mining algorithms have their limitations. To achieve more insight into these limitations the starting point in this paper is the α-algorithm [3] for which it is proved under which conditions and process constructs the algorithm works. After presentation of the α-algorithm, a classification is given of the process constructs that are difficult to handle for this type of algorithms. Then, for some constructs (i.e. short loops) it is illustrated in which way the α-algorithm can be extended so that it can correctly discover these constructs.

Journal ArticleDOI
TL;DR: This work revisits hybrid and continuous PN formalisms, all of them being in essence hybrid models, and presents “a biased” view of works in the hybrid PN arena.

Book ChapterDOI
07 Apr 2003
TL;DR: The construction improves a previous proposal by Yoneda and Ryuba of some state space abstractions for Time Petri nets and preserves their branching time temporal properties.
Abstract: This paper is concerned with construction of some state space abstractions for Time Petri nets. State class spaces were introduced long ago by Berthomieu and Menasche as finite representations for the typically infinite state spaces of Time Petri nets, preserving their linear time temporal properties. This paper proposes a similar construction that preserves their branching time temporal properties. The construction improves a previous proposal by Yoneda and Ryuba. The method has been implemented, computing experiments are reported.

Book ChapterDOI
23 Jun 2003
TL;DR: This work defines refinement operators for nets and investigates preservation of correctness through these operators, which gives rise to a class of nets that are provably correct.
Abstract: Workflow nets are recognized as a modelling paradigm for the business process modelling. We introduce and investigate several correctness notions for workflow nets, ranging from proper termination of cases to their mutual independence. We define refinement operators for nets and investigate preservation of correctness through these operators. This gives rise to a class of nets that are provably correct.

Journal ArticleDOI
TL;DR: This paper proposes a multilevel metamodeling technique with precise static and dynamic semantics (based on a refinement calculus and graph transformation) where the structure and operational semantics of mathematical models can be defined in a UML notation without cumbersome mathematical formulae.
Abstract: As UML 2.0 is evolving into a family of languages with individually specified semantics, there is an increasing need for automated and provenly correct model transformations that (i) assure the integration of local views (different diagrams) of the system into a consistent global view, and, (ii) provide a well-founded mapping from UML models to different semantic domains (Petri nets, Kripke automaton, process algebras, etc.) for formal analysis purposes as foreseen, for instance, in submissions for the OMG RFP for Schedulability, Performance and Time. However, such transformations into different semantic domains typically require the deep understanding of the underlying mathematics, which hinders the use of formal specification techniques in industrial applications. In the paper, we propose a multilevel metamodeling technique with precise static and dynamic semantics (based on a refinement calculus and graph transformation) where the structure and operational semantics of mathematical models can be defined in a UML notation without cumbersome mathematical formulae.

Journal ArticleDOI
TL;DR: This review provides a very brief summary of these interesting new research directions in the mathematical structures known as Petri Nets.
Abstract: The mathematical structures known as Petri Nets have recently become the focus of much research effort in both the structural and quantitative analysis of all kinds of biological networks. This review provides a very brief summary of these interesting new research directions.

Journal Article
TL;DR: In this article, the simi- larities between the counterparts in traditional biochemical modelling and Petri net theory are discussed, where the stoichiometry matrix of a metabolic network corresponds to the incidence matrix of the Petri nets.
Abstract: Petri net concepts provide additional tools for the modelling of metabolic networks. Here, the simi- larities between the counterparts in traditional biochemical modelling and Petri net theory are discussed. For exam- ple the stoichiometry matrix of a metabolic network corresponds to the incidence matrix of the Petri net. The flux modes and conservation relations have the T-invariants, respectively, P-invariants as counterparts. We reveal the biological meaning of some notions specific to the Petri net framework (traps, siphons, deadlocks, liveness). We focus on the topological analysis rather than on the analysis of the dynamic behaviour. The treatment of external metabolites is discussed. Some simple theoretical examples are presented for illustration. Also the Petri nets corre- sponding to some biochemical networks are built to support our results. For example, the role of triose phosphate isomerase (TPI) in Trypanosoma brucei metabolism is evaluated by detecting siphons and traps. All Petri net prop- erties treated in this contribution are exemplified on a system extracted from nucleotide metabolism.

01 Jan 2003
TL;DR: The Diagnoser Approach for discrete-event systems modeled by automata developed in earlier work is adapted and extended to on-line fault diagnosis of systems modeling by Petri nets, resulting in a centralized diagnosis algorithm based on the notion of "Petri net diagnosers".
Abstract: The problem of detecting and isolating fault events in dynamic systems modeled as discrete-event systems is considered. The modeling formalism adopted is that of Petri nets with labeled transitions, where some of the transitions are labeled by different types of unobservable fault events. The Diagnoser Approach for discrete-event systems modeled by automata developed in earlier work is adapted and extended to on-line fault diagnosis of systems modeled by Petri nets, resulting in a centralized diagnosis algorithm based on the notion of "Petri net diagnosers". A distributed version of this centralized algorithm is also presented. This distributed version assumes that the Petri net model of the system can be decomposed into two place-bordered Petri nets satisfying certain conditions and that the two resulting Petri net diagnosers can exchange messages upon the occurrence of observable events. It is shown that this distributed algorithm is correct in the sense that it recovers the same diagnostic information as the centralized algorithm. The distributed algorithm provides an approach for tackling fault diagnosis of large complex systems.

Journal Article
TL;DR: It is argued that appreciated tools from biochemistry and also low-level Petri nets can yield only part of the desired results, whereas executable high-level net models lead to a number of valuable additional insights by combining symbolic analysis and simulation.
Abstract: Computer assisted analysis and simulation of biochemical pathways can improve the understanding of the structure and the dynamics of cell processes considerably The construction and quantitative analysis of kinetic models is often impeded by the lack of reliable data However, as the topological structure of biochemical systems can be regarded to remain constant in time, a qualitative analysis of a pathway model was shown to be quite promising as it can render a lot of useful knowledge, e g, about its structural invariants The topic of this paper are pathways whose substances have reached a dynamic concentration equilibrium (steady state) It is argued that appreciated tools from biochemistry and also low-level Petri nets can yield only part of the desired results, whereas executable high-level net models lead to a number of valuable additional insights by combining symbolic analysis and simulation

Journal ArticleDOI
TL;DR: A failure prediction method for PM by state estimation using the Kalman filter is presented, which can achieve early failure detection and isolation for fault diagnosis but also facilitates event count, system state description, and automatic shutdown or regulation.
Abstract: Preventive maintenance (PM) is an effective approach for reliability enhancement. Time-based and condition-based maintenance are two major approaches for PM. In contrast, condition-based maintenance can be a better and more cost-effective type of maintenance than time-based maintenance. However, irrespective of the approach adopted for PM, whether a failure can be detected early or even predicted is the key point. This paper presents a failure prediction method for PM by state estimation using the Kalman filter. To improve preventive maintenance, this study uses a hybrid Petri-net modeling method coupled with fault-tree analysis and Kalman filtering to perform failure prediction and processing. A Petri net arrangement, viz, early failure detection and isolation arrangement (EFDIA), is used; it facilitates alarm, early failure detection, fault isolation, event count, system-state description, and automatic shutdown or regulation. These functions are very useful for health-monitoring and preventive-maintenance of a system. This study implements EFDIA to an application-specific integrated circuit on a Xilinx Demonstration Board. A condition-monitoring system of a thermal power plant is used as an example to demonstrate the proposed scheme. Linking the Kalman filter to the EFDIA Petri net, a condition-based failure prediction and processing scheme has been completed for preventive maintenance. This paper presents a failure prediction and processing scheme for PM via the thermal power-plant example, by using a hybrid Petri net modeling method endowed with fault-tree analysis and Kalman filtering. The FPN (Petri net dealing with system failure) has to be constructed beforehand. The next step is to obtain control charts for all fault places in the FPN in order to prescribe thresholds and increment times for every step in Kalman prediction. Afterwards, the system model of each place in the FPN must be derived to perform Kalman filtering. With these prerequisites, this method can be applied to any system. The proposed Petri net approach not only can achieve early failure detection and isolation for fault diagnosis but also facilitates event count, system state description, and automatic shutdown or regulation. These capabilities are very useful for health monitoring and PM of a system. Since the triggering signal of S/sub i/ place of the EFDIA in Section IV (S/sub i/ is a place for the Kalman-predicted indicator value of the sensing signal for the Petri net dealing with system failure) indicates that subsystem #i performance is going to reach the prescribed failure threshold, the signal can be provided via the Kalman filtering method in Section III. Linking the Kalman filter to the EFDIA Petri net, a condition-based failure prediction and processing scheme has been completed for preventive maintenance.

Book ChapterDOI
24 Feb 2003
TL;DR: The first steps of a new integrating methodology to develop and analyse models of biological pathways in a systematic manner using well established Petri net technologies using qualitative and quantitative analysis for behaviour prediction are demonstrated.
Abstract: Biological networks tend to be very dense and large - far beyond human skills. Therefore, a crucial point seems to be their concise and unambiguous representation to handle these highly integrated networks computationally in an efficient manner. Moreover, our knowledge about a particular pathway is generally widely spread over various separate data bases, using a quite large variety of different graphical schemes.This talk demonstrates the first steps of a new integrating methodology to develop and analyse models of biological pathways in a systematic manner using well established Petri net technologies. Petri nets represent a modelling method, very well-known for its powerful combination of readability and analyzability. They provide a generic description principle, applicable on any abstraction level. At the same time, they have a sound formal semantics, allowing thorough model evaluation. Hence, Petri nets may be used for a concise formal representation, allowing an unifying view on knowledge coming from different sources.The whole approach comprises step-wise modelling, animation, model validation as well as qualitative and quantitative analysis for behaviour prediction. In this presentation, the first phase is addressed - how to develop and validate a model.The example used in that talk is devoted to apoptosis, the genetically programmed cell death. Apoptosis is an essential part of normal physiology for most metazoan species. Disturbances in the apoptotic process could lead to several diseases. The pathway of apoptosis includes highly complex mechanisms to control and execute programmed cell death. We have intentionally chosen that example to stress the fact that even incomplete and uncertain knowledge may be subject of our technology. Actually, we are convinced that step-wise incremental modelling accompanied by running repeated analyses are the only chance to get dependable larger models.The results provide a mathematically unique and valid model, enabling the simulation of known properties as well as to check the model for self-consistency. The next steps are obvious: after being convinced of the model's integrity, we are ready to use the model for questions where the answers are not yet known.

Journal ArticleDOI
01 Mar 2003
TL;DR: A model mapping method to convert a directed network graph (DNG) based workflow model, which is built by a graphic process modeling language extended with time information, into a TCWF-net, and discusses its schedulability verification and synthesis.
Abstract: The analysis of the correctness and rationality of a workflow model plays an important role in the research of workflow techniques and successful implementation of workflow management. This paper points out the relevant problems in the verification and analysis of a workflow model. It discusses two important properties: schedulability and boundedness of a workflow model considering timing constraints. To specify the timing constraints, WorkFlow net is extended with time information, leading to timing constraint workflow net (TCWF-net). This paper presents a model mapping method to convert a directed network graph (DNG) based workflow model, which is built by a graphic process modeling language extended with time information, into a TCWF-net. It then discusses its schedulability verification and synthesis. An algorithm to decompose an acyclic and free-choice TCWF-net into a set of T-components is presented, followed by a boundedness verification method. The usefulness of the research results is illustrated by an example.

Journal Article
TL;DR: Genomic Object Net 1.0 is a software package for creating models and simulations of biopathways and provides a tool to transform biopathway models in KEGG and BioCyc to the GON XML files for modelling and simulation.
Abstract: Genomic Object Net (GON) 1.0 is a software package for creating models and simulations of biopathways. Its core architecture employs the notion of a hybrid functional Petri net with extension (HFPNe). HFPNe can seamlessly handle discrete and continuous objects and events while keeping the model components themselves simple. With the feature and graphical model editor, biopathways can be modelled intuitively and simulated on GON. The subsequent output of the simulation results can be evaluated in customised views on GON Visualizer by writing an XML file. Additionally, GON provides a tool to transform biopathway models in KEGG and BioCyc to the GON XML files for modelling and simulation. The tool avoids a lot of tedious work by users, enabling them to focus on the biological

Journal ArticleDOI
TL;DR: It is shown how a 1-safe Petri net and a requirement on the behaviour of the net can be translated into a logic program such that the bounded model checking problem for theNet can be solved by computing stable models of the corresponding program.
Abstract: In this paper, bounded model checking of asynchronous concurrent systems is introduced as a promising application area for answer set programming. As the model of asynchronous systems a generalisation of communicating automata, 1-safe Petri nets, are used. It is shown how a 1-safe Petri net and a requirement on the behaviour of the net can be translated into a logic program such that the bounded model checking problem for the net can be solved by computing stable models of the corresponding program. The use of the stable model semantics leads to compact encodings of bounded reachability and deadlock detection tasks as well as the more general problem of bounded model checking of linear temporal logic. Correctness proofs of the devised translations are given, and some experimental results using the translation and the Smodels system are presented.

Book ChapterDOI
23 Jun 2003
TL;DR: The paradigm of "nets within nets" is applied since it is well suited to express the dynamics of open, mobile systems, and the results are carried forward to model mobility in the area of agent systems.
Abstract: Mobility creates a new challenge for dynamic systems in all phases of the life cycle, like modelling, execution, and verification. In this work we apply the paradigm of "nets within nets" to this area since it is well suited to express the dynamics of open, mobile systems. The advantages of Petri nets - intuitive graphical representation and formal semantics - are retained and supplemented with a uniform way to model mobility and mobile (agent) systems. First the modelling of mobility is introduced in general, the results are carried forward to model mobility in the area of agent systems. The usefulness of the approach is shown in a second step by modelling a small case study, the implementation of a household robot system.

Journal ArticleDOI
11 Aug 2003
TL;DR: The main advantage over other methods is that the observer presented herein is given as an IPN, allowing further analysis of the system-observer pair, and a method for designing asymptotic observers is discussed.
Abstract: This paper is concerned with the analysis of the observability of the discrete event systems (DES) modeled by interpreted Petri nets (IPN). This paper presents three major contributions on the field of the observability of DES. First, an observability definition for IPN is proposed. This definition is more precise than previous ones because it deals with the possibility of determining the system's initial state, using the knowledge of the system's inputs, outputs, and structure. Later, a novel characterization of the IPN exhibiting the observability property that is based on the IPN structure is presented. Finally, a method for designing asymptotic observers is discussed. The main advantage over other methods is that the observer presented herein is given as an IPN, allowing further analysis of the system-observer pair.

01 Jan 2003
TL;DR: In this paper, the authors introduce the field of object Petri nets, and give small examples and proceeds by giving formal se- mantics, some of which are modelled within the Renew tool.
Abstract: The nets-within-nets paradigm provides an innovative mod- elling technique by giving tokens themselves the structure of a Petri net. These nets, called token nets or object nets, also support the object ori- ented modelling technique as they may represent real world objects with a proper dynamical behaviour. Between object nets and the surrounding net, called system net, various interaction mechanisms exist as well as between different object nets. This introduction into the field of object Petri nets starts with small examples and proceeds by giving formal se- mantics. Some of the examples are modelled within the formalism of the Renew tool. Finally the differences between reference and two kinds of value semantics are discussed.