Parosh Aziz Abdulla Pritha Mahata Richard Mayr Dense-Timed Petri Nets: Checking Zenoness, Token liveness<br> and<br><br> Boundedness http://arxiv.org/abs/cs/0611048 2006 CoRR abs/cs/0611048 db/journals/corr/corr0611.html#abs-cs-0611048 informal publication