Predicates for state changes vs. processes for event patterns

The two informal 'mental landscapes' that provide the intuitive substratum for state- oriented and event-oriented formal specifications are discussed, and abstractly characterised as net- works of constraints. The structuring facilities offered by the two approaches are contrasted. A technique is introduced for expanding an event-oriented specification consisting of a fixed pattern of interacting processes into a state-oriented specification formed by a complex 'action predicate' manipulating a set of state variables. Although by this transformation the event and process con- cepts can be absorbed into the state-based conceptual framework, we discuss some good reasons for regarding these concepts as primitive expressive tools, and for structuring specifications around them.