Abstract. In many approaches to the veri cation of reactive systems, operational semantics are used to model systems whereas speci cations are expressed in temporal logics. Most approaches however assume, that the initial speci cation is indeed the intended one. Changing the speci cation thus necessitates to nd an accordingly adapted system and to carry out the veri cation from scratch. During a systems life cycle however, changes of the requirements and resources necessitate repeated adaptations of speci cations. We here propose a method that supports syntactic action re nement SAR and allows to automatically obtain a priori correct systems by hierarchically adding details to the according speci cations. More precisely, w e give a de nition of SAR for formulas ' of the Modal Mu-Calculus denoted by ' ; Q that conforms to SAR for T C S P -like process terms P denoted P ; Q in the following sense: The system induced by a process term P satis es a speci cation ' if and only if the system induced by the re ned term P ; Q satis es the re ned speci cation ' ; Q . Model checking is used to decide, whether the initial system satis es the initial speci cation. If we are not satis ed with the obtained re nement P ; Q o r ' ; Q w e reuse already gained veri cation information P satis es ' that is as the basis for other re nement steps. This can be conceived as a method to reengineer systems. Syntactic action re nement allows to handle in nite state systems. Further, the system induced by P might be exponentially smaller that the system induced by P ; Q . We explain how our results can thus also be exploited to enhance model checking techniques. Finally, w e apply our results to an example.
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