Javier Esparza Reachability in Live and Safe Free-Choice Petri Nets is NP-Complete. 211-224 1998 198 Theor. Comput. Sci. 1-2 db/journals/tcs/tcs198.html#Esparza98 http://dx.doi.org/10.1016/S0304-3975(97)00235-1