Message sequence charts are an attractive formalism for specifying communicating systems. One way to test such a system is to substitute a component by a test process and observe its interaction with the rest of the system. Unfortunately, local observations can combine in unexpected ways to define implied scenarios not present in the original specification. Checking whether a scenario specification is closed with respect to implied scenarios is known to be undecidable when observations are made one process at a time. We show that even if we strengthen the observer to be able to observe multiple processes simultaneously, the problem remains undecidable. In fact, undecidability continues to hold even without message labels, provided we observe two or more processes simultaneously. On the other hand, without message labels, if we observe one process at a time, checking for implied scenarios is decidable.
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