Luca Bernardinello Nicola Bonzanni Marco Mascheroni Lucia Pomello Modeling Symport/Antiport P Systems with a Class of Hierarchical Petri Nets. 124-137 2007 conf/membrane/2007 Workshop on Membrane Computing http://dx.doi.org/10.1007/978-3-540-77312-2_8 db/conf/membrane/membrane2007.html#BernardinelloBMP07