Specification of Reactive Hardware/Software Systems - Electronic ...

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292 Process Part of POOSL 9.7.1 The Grain of Concurrency The grain of concurrency in POOSL is the processes object. All internal activities of process objects are performed sequentially. The evaluation of data expressions, for example, is always performed from left to right. Consider message-send expression E m(E1 ¥¡ ¡ ¡ ¥ En). To evaluate this expression, first destination expression E is evaluated. Then the parameters E1 ¥¡ ¡ ¡ ¥ En are evaluated from left to right and finally the message is sent to the destination object. The order in which the expressions are evaluated is formalised by rules (a) and (b) of the transition system of Subsection 8.5.3: (a) Method call 1 E e ¥ ¡ ¥ s¥ (b) Method call 2 ¡ ¥ Sys £ e e E ¥¡ ¡ ¡ ¥ m(E1 Ee ¥ s¥ n)¥ ¡ ¡ e e E ¥¡ ¡ ¡ ¥ m(E1 Ee ¡ ¡ ¥ s¡ ¥ ¡ n)¥ E e ¥ ¡ ¥ s¥ ¡ ¥ Sys £ E e ¡ ¥ ¡ ¡ ¥ s¡ ¥ ¡ ¥ Sys £ ¡ ¥ Sys E e ¡ ¥ ¡ ¡ ¥ s¡ ¥ ¡ ¡ ¡ ¥ Sys ¥ Sys ¡ ¡ m( ¤ 1 ¥¡ ¡ ¡ ¥ ¤ i 1 ¤ ¥ Ee ¡ ¡ ¥ E ¥¡ e )¥ ¡ ¥ s¥ n ¤ m( ¤ 1 ¥¡ ¡ ¡ ¥ ¤ i 1 ¥ Ee ¡ ¥¡ ¡ ¡ ¥ E e n)¥ ¡ ¡ ¥ s¡ ¥ ¥ Sys £ ¡ ¡ ¥ Sys Why did we chose to evaluate the expressions in a strict sequential manner, instead of in a concurrent one? For it seems that concurrent expression evaluation can easily be dealt with by replacing rule (b) by (b”): (b”) Method call 2 e ¡ ¡ ¥ s¥ e Ei ¡ ¥ ¡ ¡ ¡ ¥ s¡ ¥ ¡ ¥ Sys Ei ¥ e e ¥¡ ¡ ¡ ¥ E m(E1 Ee i 1 ¥ Eei ¥¡ ¡ ¡ ¥ Ee ¡ s¥ ¥ ¡ n)¥ ¥ Sys £ E e m(E e 1 ¥¡ ¡ ¡ ¥ E e i 1 ¥ Ee i ¡ ¥¡ ¡ ¡ ¥ Ee n )¥ ¡ ¡ ¥ s¡ ¥ ¥ Sys £ ¡ ¡ ¥ Sys However, if we take a closer look at this rule, we see that it does not formalise concurrent expression evaluation at all! The problem is that the rule allows n 1 different data objects to be executing one of their methods at the same time. Therefore n 1 different local variable environments have to be active at the same time. Unfortunately, there exists only one active environment and this environment is positioned at the top of stack s and belongs to the object which is currently executing one of its methods. This problem is not easily solved in our chosen type of semantics. Each possible solution will most certainly complicate the semantics in a serious way. Next to this theoretical difficulty of concurrent expression evaluation, a more practical problem exists. Consider the following definition of a (very simple and restricted) class Bit:
9.7 The Development of POOSL 293 data class Bit instance variables bit instance methods error setToZero setToOne invert primitive bit 0; bit 1; if bit 0 then bit : bit 1 self self else bit : bit ¤ 1 fi; if bit 0 or bit ¡ 1 then self error fi; self Primitive message error aborts the execution with an error message. Assume that a variable b refers to a Bit which is setToZero and consider the concurrent evaluation of expression (b invert) ¡ (b invert). It seems clear that the result of this evaluation must be true 8 . Indeed, true is one of the possible outcomes. Another possibility, however, is that the evaluation aborts with an error message, leaving b in the unexpected state where instance variable bit refers to 2! Problems of this kind are well-known in object-oriented languages, such as Smalltalk-80 [GR89], that support processes as an orthogonal language concept. These processes may act on the same collection of objects. It is even possible that they are executing the same method in the same object at the same time [AR89b]. This can result in problems of synchronisation and mutual exclusion as in the case of our Bit example. Of course, these problems of synchronisation and mutual exclusion can be solved if concurrently evaluated expressions are required to be side-effect free. However, the application of expressions with side-effects in object-oriented languages is not at all unusual. In Eiffel [Mey88], a restricted use of functions with side-effects is even recommended and exploited (see also Paragraph 6.6.3.5). The problems described above also occur when other forms of concurrency within process objects are allowed. We have therefore determined the grain of concurrency at the level of the process object. 9.7.2 Conditional Message Reception The basic process communication model in POOSL is based upon the synchronous pairwise message-passing mechanism of CCS [Mil89]. In this model, processes communicate by exchanging messages over (static) channels. If a process sends a message on a channel, it has in principle no knowledge of the identity of the receiving process. Vice versa, if a process receives a message, it has no knowledge of the senders identity. The basic communication model of POOSL thus allows the modelling of strongly distributed systems (see also Paragraph 6.4.4.2). However, many complex (object-oriented) systems are characterised by weak distribution (see Paragraph 6.4.4.1). A process in a weakly 8 Note that the primitive equality message only determines whether two expressions refer to the same object or not.
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Specification of Reactive Hardware/
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to Leny, Inge and our parents
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Contents Acknowledgements v 1 Intro
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CONTENTS ix 4.6.5 Aggregate Semanti
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CONTENTS xi 6 Modelling of Concurre
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CONTENTS xiii 6.6.7.2 System Specif
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CONTENTS xv 11.4.2.3 Requirements C
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List of Figures 1.1 Chapter Overvie
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LIST OF FIGURES xix 6.16 Strongly D
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Chapter 1 Introduction 1.1 Objectiv
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1.1 Objectives 3 understood and sti
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1.1 Objectives 5 Informal and Forma
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1.2 Thesis Organisation 7 Chapter 1
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1.2 Thesis Organisation 9 Recommend
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Chapter 2 On Specification of React
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2.3 Specifications, Models and View
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2.4 Specification Languages, Formal
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2.4 Specification Languages, Formal
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2.5 Modelling Concepts 23 entirety.
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2.5 Modelling Concepts 25 Tradition
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2.6 Activity Frameworks for Specifi
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2.6 Activity Frameworks for Specifi
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2.7 Summary 31 offer many guideline
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Chapter 3 Concepts for Analysis, Sp
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3.3 Concepts 35 3. creation of a sy
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3.5 Combining Compatible Concepts 3
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3.6 Practical Use of Concepts 39 3.
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3.6 Practical Use of Concepts 41 Th
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3.6 Practical Use of Concepts 43 ac
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3.6 Practical Use of Concepts 45 ac
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3.6 Practical Use of Concepts 47 wa
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3.6 Practical Use of Concepts 49 Ge
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3.6 Practical Use of Concepts 51 Re
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3.7 Summary and Concluding Remarks
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Chapter 4 Abstraction of a Problem
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4.1 Introduction 57 earlier phase t
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4.2 Objects 59 An object can mean t
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4.2 Objects 61 wonder what other ob
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4.3 Classes 63 A message interface
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4.3 Classes 65 Class B Attributes M
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4.4 Data Objects and Process Object
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4.5 Clusters 79 flows. Clusters ena
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4.6 Aggregates 81 Therefore a compl
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4.6 Aggregates 83 4.6.4 Clustered A
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4.6 Aggregates 85 An old philosophi
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4.7 Identity and Object Identifiers
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4.8 Properties of Composites 89 for
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4.8 Properties of Composites 91 com
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4.8 Properties of Composites 93 Whe
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4.8 Properties of Composites 95 In
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4.9 Channel Hiding 97 Servant A Sup
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4.11 Composites and Hierarchies 99
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4.12 Object Variety 101 Part class
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4.13 Object Class Models 103 Anothe
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4.13 Object Class Models 105 of obj
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4.14 Attributes and Variables 107 c
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4.14 Attributes and Variables 109 4
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4.15 Operations, Services, Methods
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4.15 Operations, Services, Methods
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4.17 Summary 115 Accidental Propert
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Chapter 5 Concepts for the Integrat
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5.2 Architecture 119 Requirements D
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5.3 Design Philosophy 121 this. The
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5.4 Essential Model and Extended Mo
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5.4 Essential Model and Extended Mo
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5.6 Architecture and Implementation
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5.7 Views 129 Management £ £ £ P
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5.7 Views 131 Behaviour Behaviour U
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5.8 Boundaries 133 5.8 Boundaries 5
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5.8 Boundaries 135 x x Object A y z
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5.8 Boundaries 137 x w w x Object A
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5.8 Boundaries 139 In Chapter 10 we
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5.9 Transformations 141 The modific
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5.10 Formalisation 143 Level 1 Leve
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5.11 Summary 145 Behaviour Requirem
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Chapter 6 Modelling of Concurrent R
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6.2 Concurrency and Synchronisation
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6.3 Communication 157 between proce
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6.3 Communication 159 or to transfo
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6.3 Communication 161 6.3.3.4 Messa
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6.3 Communication 163 Synchronous m
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6.3 Communication 165 channel-name
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6.3 Communication 167 can be descri
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6.3 Communication 169 statement aft
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6.3 Communication 171 (normalCourse
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6.3 Communication 173 ∞ client re
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6.3 Communication 175 There is a gr
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6.3 Communication 177 process A pro
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6.3 Communication 179 Clocks An int
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6.3 Communication 181 whether the s
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6.3 Communication 183 6.3.9.4 Model
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6.3 Communication 185 6.3.10.2 Desc
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6.4 Distribution 187 force to take
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6.4 Distribution 189 is that the in
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6.4 Distribution 191 links to each
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6.4 Distribution 193 6.4.6 Distribu
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6.5 Scenarios 195 entities. This su
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6.5 Scenarios 197 startJob readyToA
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6.5 Scenarios 199 collection of tex
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6.6 Dynamic Behaviour 201 selection
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6.6 Dynamic Behaviour 203 processes
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6.6 Dynamic Behaviour 205 even for
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6.6 Dynamic Behaviour 207 with the
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6.6 Dynamic Behaviour 209 6.6.4 Sep
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6.6 Dynamic Behaviour 211 6.6.5 Mod
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6.6 Dynamic Behaviour 213 to pay at
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6.6 Dynamic Behaviour 215 A method
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6.6 Dynamic Behaviour 217 to happen
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6.6 Dynamic Behaviour 219 c a Multi
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6.6 Dynamic Behaviour 221 but not t
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6.7 Real-Time Modelling 223 claimed
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6.7 Real-Time Modelling 225 in the
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6.8 Summary 227 They can graphicall
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Chapter 7 Introduction to the Seman
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7.2 A Brief Language Characterisati
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7.3 The Role of Semantics 233 7.3 T
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7.4 Mathematical Preliminaries 235
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7.4 Mathematical Preliminaries 237
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7.5 Concluding Remarks 239 0 ¡ Bin
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Page 263 and 264: 8.3 Formal Syntax 243 runk, and cun
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Page 273 and 274: 8.6 Primitive deepCopy Messages 253
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Page 277 and 278: 8.7 Example: Complex Numbers 257 is
Page 279 and 280: 8.8 Summary and Concluding Remarks
Page 281 and 282: Chapter 9 Process Part of POOSL Cha
Page 283 and 284: 9.3 Formal Syntax 263 In almost any
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Page 317 and 318: 9.7 The Development of POOSL 297 Em
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Page 321 and 322: 9.8 Summary 301 To prepare the deve
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Page 325 and 326: 10.2 Instance Structure Diagrams 30
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Page 329 and 330: 10.3 Some Properties of Transformat
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Page 351 and 352: 10.8 Summary 331 are transformation
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Page 355 and 356: 11.2 SHE Context and Phases 335 Inf
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Page 361 and 362: 11.3 SHE Framework 341 the system.
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11.4 Essential Specification Modell
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11.5 Extended Specification Modelli
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Chapter 12 Case Study Chapter 1 Cha
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12.2 Initial Requirements Descripti
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12.3 The Essential Specification 37
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12.4 Towards an Extended Specificat
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12.5 Concluding Remarks 399 i1 DIST
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12.5 Concluding Remarks 401 Feeder
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Chapter 13 Conclusions and Future W
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13.2 P.H.A. van der Putten’s Cont
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13.2 P.H.A. van der Putten’s Cont
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13.4 Related Results 409 The notion
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Appendix A The POOSL Transition Sys
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A.1 The Data Part of POOSL 413 (12)
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A.2 The Process Part of POOSL 415 A
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A.2 The Process Part of POOSL 417 (
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A.2 The Process Part of POOSL 419 (
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A.2 The Process Part of POOSL 421 i
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Appendix B Proofs of Propositions a
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Proofs of Propositions and Transfor
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Appendix C Glossary of Symbols This
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Glossary of Symbols 437 267 Cp ¥¡
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References [AB90] P. America and F.
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REFERENCES 441 [C [C [C 86] B. Cohe
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REFERENCES 443 [Her90] A.J.H. Herbe
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REFERENCES 445 [MV93] S. Mauw and G
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REFERENCES 447 [vE89] P. van Eijk.
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Index abort, 297 abstract action so
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INDEX 451 strong, 190, 293 subsyste
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INDEX 453 delay, 178 do-statement,
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Summary This combined thesis descri
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Samenvatting Dit gecombineerde proe
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Curricula Vitae Piet van der Putten
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