The Good, the Bad and the Ugly: Well-Formedness of Live Sequence Charts

The Life Sequence Chart (LSC) language is a conservative extension of the well-known visual formalism of Message Sequence Charts. An LSC specification formally captures requirements on the inter-object behaviour in a system as a set of scenarios. As with many languages, there are LSCs which are syntactically correct but insatisfiable due to internal contradictions. The authors of the original publication on LSCs avoid this problem by restricting their discussion to well-formed LSCs, i.e. LSCs that induce a partial order on their elements. This abstract definition is of limited help to authors of LSCs as they need guidelines how to write well-formed LSCs and fast procedures that check for the absence of internal contradictions. To this end we provide an exact characterisation of well-formedness of LSCs in terms of concrete syntax as well as in terms of the semantics-giving automata. We give a fast graph-based algorithm to decide wellformedness. Consequently we can confirm that the results on the complexity of a number of LSC problems recently obtained for the subclass of well-formed LSCs actually hold for the set of all LSCs.