Roberto Barbuti Andrea Maggiolo-Schettini Paolo Milazzo Simone Tini Compositional semantics and behavioral equivalences for P Systems. 77-100 2008 395 Theor. Comput. Sci. 1 http://dx.doi.org/10.1016/j.tcs.2008.01.028 db/journals/tcs/tcs395.html#BarbutiMMT08