A modelling scheme is presented which allows the abstract description of a collection of concurrent processes (a subsystem). A model in the scheme provides a specification of a subsystem which describes its behavior in relation to other subsystems but hides the subsystem’s operational detail. A model consists of a definition of the subsystem’s interface, a procedural definition of the legal usage of the interface and a non-procedural description of the legal uses of the subsystem over time. Models in the scheme are rigorous, unambiguous specifications of the components within a software system. The models may be used to guide the implementation of the components or the formulate arguments as to the appropriateness of a system’s design
