Jean-Michel Couvreur Sébastien Grivet Denis Poitrenaud Unfolding of Products of Symmetrical Petri Nets. 121-143 2001 conf/apn/2001 ICATPN http://link.springer.de/link/service/series/0558/bibs/2075/20750121.htm db/conf/apn/icatpn2001.html#CouvreurGP01