We consider the problem of precisely deening UML active classes with an associated state chart. We are convinced that the rst step to make UML precise is to nd an underlying formal model for the systems modelled by UML. We argue that labelled transition systems are a sensible choice; indeed they have worked quite successfully for languages as Ada and Java. Moreover, we think that this modelization will help to understand the UML constructs and to improve their use in practice. Here we present the labelled transition system associated with an active class using the algebraic speciication language Casl. The task of making precise this fragment of UML raises many questions about both the \precise" meaning of some constructs and the soundness of some allowed combination of constructs.
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