R. Freund, L. Kari, M. Oswald, P. Sosik:
"Computationally Universal P Systems without Priorities: Two Catalysts are Sufficient";
Theoretical Computer Science, 330 (2005), 251 - 266.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05936147