Expressive flexibility of a constraint-oriented notation

We illustrate the expressive flexibility of the 'co-notation', a declarative, structured, constraint-oriented specification technique for concurrent systems where constraints on actions and on data variables are intertwined. Flexibility is intended here as the ability to closely match the structure of the initial, informal behavioural description of the system, and to directly formalize the conceptual links among data variables and events, and their groupings, as expressed in that description, We contrast the co-notation with two related and 'mature' specification techniques, namely predicate/transition Petri nets and LOTOS, also by means of a running example. The Petri net approach supports a quite direct specification of the links among actions and data variables, but it lacks structuring facilities; LOTOS does support encapsulation, but its process-oriented view does not offer full flexibility in defining and composing groups of those conceptual links. We show that the simple yet powerful constraint conjunction operator of the co-notation, supporting both shared-action (rendez-vous) and shared-variable interaction mechanisms, achieves a higher expressive flexibility than the other models, and brings to a full exploitation the constraint-oriented reasoning partially offered by LOTOS.