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The Business Process Execution Language for Web Service, known as BPEL4WS, more recently as WS-BPEL (or BPEL for short) [1], is a process definition language geared towards Service-Oriented Computing (SOC) and layered on top of the Web services technology stack. In BPEL, the logic of the interactions between a given service and its environment is described as a composition of communication actions. These communication actions are interrelated by control-flow dependencies expressed through constructs close to those found in workflow definition languages. In particular, BPEL incorporates two sophisticated branching and synchronisation constructs, namely “control links” and “join conditions”, which can be found in a class of workflow models known as synchronising workflows formalised in terms of Petri nets in [3].