Non sequential semantics for contextual P/T nets

. The problem of finding a true concurrent semantics for contextual P/T nets has not been deeply studied yet. The interest for such a semantics has been renewed by some recent proposals to equip mobile process algebras with a net semantics based on this model. In [3] we proposed a causal semantics for such nets, in this paper we study the non sequential semantics of contextual P/T nets. The semantics is based on a suitable notion of occurrence net and, equipped with a notion of history preserving bisimulation, is more discriminating of the contextual one in general, whereas both are equivalent if we restrict our attention to positive context only. 1 Introduction Place/Transition nets [13] are a well known model used to describe concurrent systems. Recently the interest in contextual nets, i.e. nets that are able to test for presence and absence of tokens, besides the usual flow relation, has grown, mainly due to the use of contextual nets to give a net semantics to the ß-calculus ([8..