The imprecision of the definitions of UML2.0 sequence diagrams, given by the Object Management Group (OMG), does not allow the obtention of all the possible valid behaviours for a given distributed system, when communicating objects are independent. We choose the causal semantics, which is suitable for this kind of systems; we propose its extension to support complex behaviours, expressed with combined fragments. We propose the implementation of our approach with Event-B in order to check later on some properties of safety, liveness and fairness.