Matthias Werner Louchka Popova-Zeugmann Jan Richling A Method to Prove Non-Reachability in Priority Duration Petri Nets. 351-368 2004 61 Fundam. Inform. 3-4 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=61&issue=3&spage=351 db/journals/fuin/fuin61.html#WernerPR04