Dezyne: Paving the Way to Practical Formal Software Engineering

Dezyne: Paving the Way to Practical Formal Software Engineering


Designing software that controls industrial equipment is challenging, especially due to its inherent concurrent nature. Testing this kind of event driven control software is difficult and, due to the large number of possible execution scenarios only a low dynamic test coverage is achieved in practice. This in turn is undesirable due to the high cost of software failure for this type of equipment.

In this paper we describe the Dezyne language and tooling; Dezyne is a programming language aimed at software engineers designing large industrial control software. We discuss its underlying two layered and compositional approach that enables reaping the benefits of Formal Methods, hereby strongly supporting guiding principles of software engineering. The core of Dezyne uses the mCRL2 language and model-checker (Jan Friso Groote to verify the correctness and completeness of all possible execution scenarios.

The IDE of Dezyne is based on the Language Server Protocol allowing a smooth integration with e.g., Visual Studio Code, and Emacs, extended with several automatically generated interactive graphical views. We report on the introduction of Dezyne and its predecessor at several large high-tech equipment manufacturers resulting in a decrease of software developing time and a major decrease of reported field defects.

1 Introduction

Designing software that controls industrial equipment is challenging. The complexity of such equipment is ever growing and so is the demand on its software. The software must control and monitor many processes in parallel. Designing the regular control flow of the processing steps is already complex, but many exceptional conditions may occur which disrupt the regular control flow and must be handled appropriately.

Testing event driven control software is difficult. Testing time on hardware is expensive so often software simulators on various levels of (hardware) details are created to mitigate these costs. However, the ultimate test is still the execution on the real hardware; errors found during the final testing are very costly and time consuming to fix. Writing test cases for (parts of) the software is difficult since, as explained earlier, many different execution scenarios have to be considered which depend on different timing conditions. As a result, only a small percentage of the possible execution scenarios is effectively tested in practice.

This kind of high-tech equipment is extremely expensive, for instance the Extreme Ultraviolet (EUV) lithography machine of ASML costs around $120 million dollar. Manufacturers of such kind of equipment are required to deliver a high uptime of their machines. In some cases, the manufacturers are penalized when a machine does not reach the agreed availability. As a result, down time of these machines as the
result of software failure must be avoided at all cost.

2 Formal Methods

As stated in the previous section, designing and testing control software of industrial high-tech equipment in a traditional fashion has serious short comings, while on the other hand the cost-of-non-quality is extremely high.

Formal Methods have been around for many decades and have been shown to deliver reliable and safe software. However, Formal Methods come with their own challenges when applied to industrially sized systems and processes. The tools and their associated languages require Formal Methods specialists. Furthermore, a translation must be made from the (informal) requirements to some description written in a Formal Methods Language. Typically, it is challenging to validate that the intended requirements are correctly captured or are that the requirements are appropriate in the first place.

The artifacts of the formalization process are typically used by Formal Methods toolsets in proving the consistency of the requirements and any of the derived refinements of the specifications. However, ultimately we need to obtain running code on the hardware for which the specified properties provably hold.

The total state space of any industrial size machine controlling application is vast and cannot be handled by any Formal Methods tool in its entirety. A practical way to handle this is to take a compositional approach: at a certain level one should be able to abstract away from details which are not relevant for the next level. This way the state-explosion problem can be avoided.

3 Dezyne

Verum has developed Dezyne [1]. Dezyne is a programming language aimed at software engineers designing large industrial control software. The language is designed to have a very low barrier to entry for regular software engineers. The Dezyne language consists of two declarative language constructs: the so-called “guarded” statement and the “on” statement for specifying the states and trigger events of a state machine, respectively. Furthermore, the Dezyne language has de facto standard imperative language constructs as present in regular languages like C or Java: variable declaration, assignment, function declaration, function call, and control flow constructs like if-then-else.

3.1 The Component Model of Dezyne
Dezyne has the notion of three models: interfaces, components, and systems.

An interface model contains the definition of a set of events. In Dezyne events are implemented as function calls. Each event declaration has a direction, “in” or “out”, and a type signature. The type signature of an event specifies the data parameters carried by an event, and the type of the return value of an event. The allowed return types are: “void”, “boolean”, enumeration types, and integer-range types. The types of the data parameters can be any arbitrary type. This is an example of event declarations of an interface:

interface IDevice {
  in void turnon();
  in void turnoff();

An interface also has a behaviour section. The behaviour of an interface prescribes the order in which events are expected to occur. A behaviour can specify additional state variables for keeping track of the state of the interface. Using the simulator of the Dezyne tooling, the user can produce sequence diagrams depicting allowed sequences of events for that interface.

An interface plays the role of a contract between components: it specifies how a client component via its requires port should interact with the underlying component providing the required functionality via its provides port. This is an example of a simple interface that consists of two events “turnon” and “turnoff”. The behaviour of the interface specifies that the events should be used alternatingly:

interface IDevice {
  in void turnon();
  in void turnoff();
  behaviour {
    enum State {On, Off};
    State s = State.Off;
    [s.Off] {
      on turnon: s = State.On;
      on turnoff: illegal;
    [s.On] {
      on turnon: illegal;
      on turnoff: s = State.Off;

Note that the statement “on turnoff: illegal;” defines that the event “turnoff” is not allowed in that state.

A component model and a system model start by declaring their provides ports and requires ports. A port is a instance of an interface.

A component model also specifies a behaviour, i.e. the actual implementation of the component. The behaviour describes how a component implements each of the events of its provides ports hereby possibly using some of its requires ports. A behaviour can specify additional variables to keep track of the state of the interaction between the component and its ports. In this way the user specifies a finite state-machine describing the implementation of the component.

This is an example of a component model:

component Fork {
  provides IDevice p;
  requires IDevice r0;
  requires IDevice r1;
  behaviour {
    enum State {On, Off};
    State s = State.Off;
    [s.Off] {
      on p.turnon(): {r0.turnon(); r1.turnon(); s = State.On;}
    [s.On] {
      on p.turnoff(): {r0.turnoff(); r1.turnoff(); s = State.Off;}

Next to the provides and requires ports, a system model lists component instances and the bindings between the ports of the component instances and the external ports of the system. These bindings determine the routing, i.e. call graph, of the events of the external ports of the systems and the component instances. This is an example of a system model:

component System {
  provides IControl cntrl;

  system {
    ProcessController controller;
    Load loader;
    Processor processor;
    Unload unloader;

    cntrl <=> controller.cntrl;
    controller.load <=> loader.load;
    controller.process <=> processor.process;
    controller.unload <=> unloader.unload;

The Dezyne tools can generate a diagram from the system model as shown in this figure:

Dezyne system diagram

Section 6 explains in more detail how these diagrams are generated. Note that each connection in the diagram corresponds to a binding statement “<=>” in the system model text.

3.2 Verifying Dezyne Models
The Dezyne language is statically checked on the use of syntax, types and an extensive list of well-formedness properties.

During the verification phase the Dezyne verification engine checks interface and component models. The verification engine translates the Dezyne model to an mCRL2 description [5, 2, 1]. Next, the mCRL2 tool set is used to convert the mCRL2 description to an LTS (labeled transition system). An LTS is a graph where nodes relate to states of the Dezyne model and each edge is labeled with an event instance. The graph describes the total state space of the model and all possible execution scenarios that cover this state space.

Both an interface and a component model are verified for the absence of deadlocks and livelocks, and whether all integer assignments are in range by inspecting the generated LTS. Additionally a component model is verified to correctly interact with its requires ports as specified by their interfaces. For this, the LTS is checked for the absence of the “illegal” label, where the presence would indicate that either the component performed an action on one of its requires port which was disallowed by the corresponding interface behaviour, or that a requires port performed an event on the component which was disallowed by the component behaviour.

Finally, the compliance of a component with all of its provides ports is verified by means of the ltscompare tool of mCRL2 using the Failures Refinement preorder relation known from CSP [6, 13, 8]. It is used to verify that the LTS of the component after hiding all internals, i.e. those events not observable by the clients using the provides ports, is a refinement of the LTS of the provides ports, i.e. for component C with provides port of type I and requires port type J it verifies that:

(C\|J) \! \upharpoonright \! \alpha (I) \, \sqsupseteq_{F} \, I

where \sqsupseteq_{F} denotes the Failures Refinement relation of CSP, \alpha(I) denotes the alphabet of process I, \| denotes the parallel composition synchronising on the common alphabet, and \upharpoonright denotes the projection operator, i.e. the complement of the hiding operator, i.e. P\!\upharpoonright\!A = P\setminus\overline{A}.

If an error is found a counter example is produced. This is a trace of events leading up to the state where the inconsistency emerges.

Next in the Dezyne verification pipeline, the trace is fed to the Dezyne simulator together with the original Dezyne models. The Dezyne simulator reconstructs the relevant state and location information such that the user can relate the counter example back to the Dezyne model text. The Dezyne simulator outputs an encoding of a sequence diagram. This encoding also allows navigating the source code location of each event occurrence in the sequence diagram. This sequence diagram is fed to a graphical engine implemented in JavaScript using a graphics library for rendering the sequence diagram. The Dezyne simulator also outputs all possible valid next events. In the sequence diagram these events are shown as buttons on the corresponding life lines. This is an example of a sequence diagram:

Dezyne sequence diagram

3.3 Two Level Approach
As described in the previous section, when verifying a Dezyne model a user does not need to have a knowledge about Formal Methods or the mCRL2 language: under the hood the Dezyne models are translated to mCRL2 models which are verified using the mCRL2 tooling and a LTS analysis tool. If a problem is found, the counter example trace is fed to the Dezyne simulator that generates an interactive sequence diagram for the user. The user can use this diagram to interactively find the root cause. The sequence diagram shows what the component is doing from the start of the system up to the reported violation. If the user has diagnosed the problem and has fixed the Dezyne models, they can rerun the Dezyne verification engine and see whether the issue has disappeared, and if not, iterate the steps mentioned above until the issue has been fixed. After which, possibly another issue is found by the verification engine and the attention is shifted to resolve this issue until no more issues remain.

3.4 Generating Verified and Reliable Code
When the Dezyne models are verified and are correct, the models are used for generating production quality machine code. Currently the Dezyne tooling has code generators for the languages C++, C#, JavaScript and GNU Guile Scheme.

The operational semantics of the Dezyne models as specified by the modeling in mCRL2 [1] and the implementation of Dezyne simulator is designed to capture the intended execution behaviour of the corresponding generated machine code. For Dezyne it is essential that the three embodiments of the semantics of Dezyne are equivalent. Given that equivalence, it follows that if no problems are found during verification, we know that all the verified properties also hold for the generated machine code.

The equivalence between the semantics used during verification and the simulation is asserted by means ltscompare-ing the trace equivalence of the underlying LTSses for a large set of test models. Thus, for each model M of this test set we check:

LTS_V(M) \, \equiv_T \, LTS_S(M)

where LTS_V denotes the LTS produced by the mCRL2 tooling as used during verification and LTS_S denotes the LTS produced using the simulator by calculating the transitive closure of the single step function of the simulator, and \equiv_T denotes the Trace Equivalence relation of CSP, i.e. L_0 \equiv_T L_1 iff traces(L_0) = traces(L_1).

The equivalence of LTS_V(M) with the generated code for component M is verified in the following way. For LTS_V(M) a trace graph cover is calculated, i.e. a set of traces such that all edges of LTS_V(M) occur in the trace set. Next to the generated code for component M, a simulation stub is also generated. The generated stub processes the textual representation of a trace: for an inward going event the corresponding implementation of the event is called. All outward events are logged and matched with the events of the trace being processed. For all traces of the LTS graph cover, the generated code plus stub is executed. If all traces of LTS_V(M) are correctly handled by the generated machine code for component M, we conclude that

traces(LTS_V(M)) \, \subseteq \, traces(CODE_L(M))

where CODE_L(M) denotes the generated code and stub in language L for model M, and traces on code denotes the set of traces the execution of the code would accept as described above.

Since LTS_V(M) is complete, i.e. for all stable states, all events are present, it follows that CODE_L(M) is also complete, and since the generated code is deterministic, i.e. for a given input event, only one response is generated, we know that the generated code cannot exhibit more behaviour as tested by the generated traces. In other words, it is that case that:

\begin{align} & traces(LTS_V(M)) \, \subseteq \, traces(CODE_L(M)) \\ \Rightarrow \,\,\,\,\,&\\ & traces(LTS_V(M)) \, = \, traces(CODE_L(M)) \end{align}

Hence, we verify using ltscompare and the running of all generated traces that

LTS_V(M) \, \equiv_T \, LTS_S(M) \, \equiv_T \, CODE_L(M)

for all of the model M of the test set, for a given language L.

The above mentioned test set covers all aspects of the Dezyne language and expected feature interactions. This test capability is also available for our users to allow them to verify above equivalence of their own (proprietary) set of models. As a result, the user can assure that if no problems are found during verification, all the verified properties hold for the generated machine code for their specific models in their environment, i.e. target compiler and platform.

4 Component Based Providing a Compositional Approach

As described above, a compositional approach to avoid the state explosion problem is highly desirable to be able to verify large complex system designs. Dezyne is a component based method: a component specifies its provides and requires ports where ports are instances of interfaces. A Dezyne interface has a contract, i.e. its behaviour, specifying the contract between two components. The interfaces of the provides ports of a component are the abstraction of said component. When designing a Dezyne component, a user does not need to know which implementation is going to provide an interface; it is sufficient to know the interface.

This provides a natural way of decomposing a larger system into smaller manageable parts. When presented with the challenge to devise certain functionality, one may approach this task by distinguishing the separate responsibilities of that functionality which be separately assigned to one or more components. The responsibility of a single component starts with its provides ports and ends with its requires ports. When designing a component one can, through verification, justly assume that the underlying components implements the interfaces of its provides ports faithfully.

The approach of interfaces as an abstraction of components not only allows the engineer to divide and conquer a problem by considering each responsibility of a component one at the time, but also is a solution to verify complete systems compositionally. This is due to an important property of the Failures Refinement relation of CSP, which allows verifying each individual component to be correct and therefore knowing that the overall system is also correct. It is the case that when two components refine their provides port interfaces then the compound of two components will also refine the top level provides port since the Failures Refinement relation is transitive and the hiding operator is congruent. We have, for component C with provides port of type I and requires port of type J, and component D with provides port of type J and requires port of type K, that:

(C\|J) \! \upharpoonright \! \alpha (I) \, \sqsupseteq_{F} \, I\,\,\, \wedge \,\,\, (D\|K) \! \upharpoonright \! \alpha (J) \, \sqsupseteq_{F} \, J \,\,\,\,\, \Rightarrow \,\,\,\,\, (C\|D\|K) \!\upharpoonright\!\alpha (I) \, \sqsupseteq_{F} \, I

Hence, for Dezyne, if we have proven that all components are correct, we have proven that the complete system is correct.

5 Guiding Principles of Software Engineering

The development of the Dezyne methodology and language design are guided by the fundamental software engineering principles as mentioned in e.g. [4]. These engineering principles are: rigor and formality, separation of concerns, modularity, abstraction, anticipation of change, generality and incrementality. We strive to have a methodology and language in supporting the user as much as possible in practicing these engineering principles. Next we discuss each of the principles in more detail and how they are reflected in Dezyne in supporting the user.

Rigor and formality
Following this principle will add precision to and increases confidence in the outcome of the creative process of constructing software by expressing requirements and ideas succinctly using a well understood notation which expresses intent and specifically allows communicating information.

As mentioned before, the Dezyne language has a rigorously and formally defined operational semantics having three different but equivalent embodiments: the modeling in mCRL2, the Dezyne simulator, and the behaviour of the generated machine code.

For the user defining a Dezyne interface describing the interaction between two components forces the user to clearly specify the expected interaction for all possible scenarios thus also for exceptional cases. The verification engine will find for a Dezyne component whether there are missing cases or whether there are states where events are not handled as they are intended.

Separation of concerns
The principle of separation of concerns is a form of divide and conquer of the work itself as well as dealing with the complexity of problems in general. By explicitly distinguishing one aspect from another, we can avoid having to deal with the compounded problem.

As mentioned before Dezyne is component based and guides the user in finding a proper decomposition of the problem, i.e. describing different aspects in behaviours independently in separate interfaces, component, and system models.

Modularity is a structural and behavioural separation of concerns by encapsulation, abstraction and (de)composition. Interfaces represent and encapsulate the entire interactional conversation between peer components. Components encapsulate all of the coordination across its ports. Systems encapsulate component instances and their inter-connections.

The principle of incrementality allows for stepwise refining solution to challenges, thereby avoiding the problems associated with a big bang approach. Verification and code generation can be used throughout the life cycle of every model. Also models can be extended aspect by aspect and hereby support an agile software development approach.

Abstraction allows us to distinguish between key issues and side issues. The key issues must be tackled early in the development process and will allow to postpone dealing with the details at the last responsible moment.

Dezyne interfaces are abstractions of component interactions and hide component implementation details and as mentioned before are key in having a compositional approach.

Awareness of generality allow us to move from a point solution to a general solution, which in turn caters for avoiding (future) work by using the general solution.

In Dezyne, interfaces represent the client’s perspective on the implementation. Taking the different point solutions under consideration, this allows deriving a general interface by capturing the commonality in the interaction of different clients.

Anticipation of change
Change is inevitable, therefore we must both keep track of all of the artifacts of the creative process, as well as maintain the consistency across the refinement steps. We must also maintain their malleability.

In Dezyne the language and the verification create the freedom to evolve any aspect while maintaining the consistency during the entire development lifecycle. If a change breaks the consistency the verification engine will report this and the user can take appropriate action.

6 Dezyne IDE Based on LSP

The core of the Dezyne tooling consists of the verification engine, simulator and code generators which are implemented using GNU Guile Scheme. In order to support a multitude of different IDEs we have implemented a language server implementing the Language Server Protocol (LSP). LSP allows clients to remain language-agnostic and share a single language-specific server implementation. Currently the Dezyne LSP server provides code completion and code navigation. The LSP server shares the parsing part with the core Dezyne tooling and as such only a single language front-end has to be maintained. When the Dezyne language is extended, it becomes immediately available for all IDEs by means of the LSP server.

Currently there are LSP client implementations of both Visual Studio Code and GNU Emacs available. Due to the nature of LSP supporting the LSP clients of other IDEs should involve little work, if any.

LSP is text oriented and currently does not provide any graphical support. To integrate the interactive Dezyne diagrams, there is a daemon process running on the machine of the user which among other things implements a webserver. For all the Dezyne views a webbrowser is started that connects to this local webserver. The editor also connects to this webserver: when the user clicks on an element in the graphical view that contains source code location, a message is sent from the webpage to the editor that contains a command for letting the editor jump to the corresponding location in the Dezyne model. Note that large parts of the daemon and JavaScript as part of a webpage rendered by the webbrowser is produced using Dezyne.

Dezyne currently has a state diagram view, a system view depicting the composition of a system model out of its subcomponent, and as mentioned before the sequence diagram for the interaction with the simulator. The user can either start the simulator directly, or the simulator is started when the verification engine finds a problem and generates a counter example. The next possible events are also depicted in the sequence diagram and if the user selects a next event, this request is send from the webpage to the daemon, and the daemon reruns the simulator with the current trace of events extended with the requested event.

Visual Studio Code with Dezyne LSP and extended with a interactive graphical view

7 Application of Dezyne in Industry

Dezyne and its predecessor ASD have been intensively used by several Verum customers for more than a decade. Verum customers are mainly high tech equipment manufacturers; their binding factor is that the cost-of-non-quality is high in the market they operate. Philips Healthcare is one of the first customers and has the longest experience in using Verum products. For there initial projects the impact of using this kind of low-entry Formal Methods based tooling has been investigated [7, 12, 11, 10] and reported that the use of the tool eliminated design errors earlier in the design process and resulted in reduced development time and an ten fold reduction in reported errors.

ASML is one of the leading manufacturers of semiconductor chip-making equipment, and has been using our tooling in different projects for different machines for some years now [9, 3, 14]. They also reported a similar decrease in field defects and decrease in development time.

The most striking difference between ASD and Dezyne is that ASD has its own proprietary Microsoft Excel resembling editor and is as such not language based. This has had all kind of implications for incorporating ASD into existing software development toolchains. For example, version management tooling is predominantly text based. This alone was experienced as a substantial obstacle in the acceptance of ASD. The feedback from users and the noted shortcomings have served as primary input to design the Dezyne language and its tooling.

Dezyne is being used by other customers which names we cannot disclose, operating in the field of semiconductor equipment industry, electronic analytical instruments, and egg grading, packing and processing machines.

8 Conclusions and Future Directions

Dezyne and its predecessor are being used at several high-tech equipment manufacturing multinationals. So far several millions lines of generated code are running in production around the world.

Software engineers without a formal methods background are productive and comfortable due to the familiar concepts Dezyne is built on. Furthermore the Dezyne language is designed to promote guiding principles of software engineering. In order to avoid the state explosion problem it is especially important to divide and conquer complexity in manageable sub-components, by introducing internal Dezyne interfaces that capture the responsibilities of these sub-components. The Dezyne verification engine guides the user in achieving an intuitive understanding of the complexity of the problem at hand and provides the user with input as to which parts of the system are candidates to be broken up.

The introduction of Dezyne and its predecessor has resulted in a decrease in software development time, a vast reduction in integration time, and a major decrease of reported field defects. This shows that Formal Methods can bring a lot of added value to industrial software engineers, provided that the method hides the underlying complexities, can be used compositionally, generates production quality and verified machine code, is properly packaged in a familiar IDEs and graphical interactive views, and promotes the application of proper software engineering principles.

Currently, the Dezyne simulator is restricted to a single component since this is used to debug the counter examples found by the verification engine. We are extending the simulator to bring multiple components in scope and even complete systems. This will further support the user in validating their systems, i.e. to check that the intended use cases and user stories are present in the behaviour of system.

We aim to extend the verification engine to prove properties on several components combined. The challenge here is to find a proper balance between the expressiveness of the language defining these properties and the clarity of the expressed properties such that a users can assess whether the formal properties capture the intended informal requirements.

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