Mojmír Kretínský Vojtech Rehák Jan Strejcek Petri nets are less expressive than state-extended PA. 134-140 2008 394 Theor. Comput. Sci. 1-2 http://dx.doi.org/10.1016/j.tcs.2007.12.003 db/journals/tcs/tcs394.html#KretinskyRS08