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Petri nets have been widely used in the past to model vari&s types of concurrent systems, including real-time systems. To do so, a variety of ad-hoc solutions have been proposed in order to allow time to be taken into account in specifications. In this paper, we initially survey these proposals and then we illustrate a new proposal (TEI nets), that unifies and generalizes all pre, vious ones. Next, we discuss how these and possibly other enrichments of the original model can be formally defined. This will be do, ne in terms of a high-level net model, called ER nets. The ability of ER nets to express a variety of behaviors justifies our choice to use them as a kernel formalism of a specification support environment that we are currently developing. In this environment, it will be possible to define new graphical notations for formal specifications and give them formal semantics in terms of the underlying ER model. This allows the …