P. Sosik, R. Freund:
" P Systems Without Priorities are Computationally Universal";
in: "Membrane Computing. International Workshop, WMC-CdeA 2002, Curtea de Arges, Romania", R. Freund (ed.); Springer, 2003, ISBN: 3-540-00611-7, 400 - 409.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC04404614