  In this article we present the {\em Constraint Aggregation Language} (CAL), a
declarative language for describing properties of stateless program components
that interact by exchanging messages. CAL allows one to describe functional as
well as extra-functional behaviours, such as computation latency. The CAL
language intention is to be able to describe the behaviour of so-called boxes
in the context of S-Net. However, the language would find application in other
coordination models based on stateless components.
