Fabio Balduzzi Angela Di Febbraro Alessandro Giua Carla Seatzu Decidability results in First-Order Hybrid Petri Nets. 41-57 2001 11 Discrete Event Dynamic Systems 1-2 http://dx.doi.org/10.1023/A:1008383031624 db/journals/deds/deds11.html#BalduzziFGS01