R. Freund, A. Leporati, M. Oswald, C. Zandron:
"Sequential P Systems with Unit Rules and Energy Assigned to Membranes";
in: "Machines, Computations, and Universality", M. Margenstern (ed.); Springer, LNCS, Berlin, 2005, ISBN: 3-540-25261-4, 200 - 210.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05936360