We discuss some of the problems concerning bisimulation relations over behaviour expressions representing non-sequential systems. The kind of bisimulation we investigate is derived from the notion of bisimulation relation between two Kripke structures, which provides a full characterisation of semantical equivalence of such structures w.r.t. the CTL* branching time temporal logic without the next-time operator. This new bisimulation on behaviour expressions, called CTL*-bisimulation, induces a congruence on recursion-free behaviour expressions, and in this paper we derive a sound and complete axiom system for this congruence.
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