1 What COSY Is and What It Is For.- 1.1 Introduction.- 1.2 Concepts, Objectives and Design Decisions.- 1.3 Structure of Bookevevr.- 1.4 Acknowledgements.- 2 Formal Theory of Basic COSY.- 2.1 Basic COSY Syntax and Semantics.- 2.2 VFS Semantics of COSY.- 2.3. Petri Net Semantics of COSY.- 2.4 Adequacy Properties of Path Programs.- 2.5 Execution Semantics for COSY.- 2.6 Semantics of COSY with Priorities.- 2.7 Bibliographic Notes.- 3 High-level COSY Programs and System Design.- 3.1 High-level COSY Syntax and Semantics.- 3.2 The Process Notation.- 3.3 Macro Generators for COSY Notation.- 3.4 The Semantics of Macro COSY Programs.- 3.5 The COSY Environment.- 3.6 The COSY System Dossier.- 3.7 Bibliographical Notes.- 4 COSY Applications.- 4.1 Two-Way Channel with Disconnect.- 4.2 The Hyperfast Banker.- 4.3 Cigarette Smokers.- 4.4 Merlin-Randell Problem of Train Journeys.- 4.5 Transforming Sequential Systems into Concurrent Systems.- 4.6 Modelling N-Modular Redundancy.- 5 Comparison of COSY with Other Models.- 5.1 COSY and CCS.- 5.2 COSY and CSP.- Full COSY and Petri Nets.- 5.4 Vector Sequences and Mazurkiewicz Traces.- 5.5 COSY and Synchronized Behaviours.- 6 Historical Perspective.- 6.1 Introduction.- 6.2 Conceptual and Methodological Framework of COSY Approach.- Appendices.- A Algebra of Relations.- B Automata and Formal Language Theory.- B.1 Strings.- B.2 Languages.- B.3 Regular Expressions and Languages.- B.4 Finite State Automata and Grammars.- C Elements of Graph Theory.- D Proofs of Theorems 2.25, 2.26 and 2.28.- E Proofs of Theorems 2.37 and 2.38.- F Proof of Theorem 2.29.- G Proof of Theorem 4.3.- H Basic COSY Notations and Macro COSY Notation.- List of Figures.- List of Algorithms.- List of Definitions.- List of Theorems.- List of Corollaries.- List of Lemmas.