Morgan Magnin Didier Lime Olivier H. Roux Symbolic State Space of Stopwatch Petri Nets with Discrete-Time Semantics (Theory Paper). 307-326 2008 Petri Nets http://dx.doi.org/10.1007/978-3-540-68746-7_21 conf/apn/2008 db/conf/apn/pn2008.html#MagninLR08