Constraint-oriented specification style and notation

We introduce an elementary and versatile notation meant to support the specification of concurrent, reactive and information systems in a so called constraint-oriented style. The co-notation has been in part inspired by the LOTOS specification language, and in part by the Z notation. Indeed, these two specification techniques are largely orthogonal and somehow complementary, the former being centred on the notions of action and process, the latter on state variables; however, limited to their respective focuses, each of them can support some constraint-oriented reasoning. In the co-notation actions and state variables are brought together into a unique framework, and promoted as equally important descriptive elements: constraint-oriented reasoning can then be uniformly applied to both. A system behaviour is characterised by sets of actions and state variables, and described as a hierarchical composition of elementary constraints and compound constraints on these elements. Interestingly, the co-notation can be presented also as a layer on top of Prolog, and has indeed been implemented in this way. The original syntax is minimal.