Gutiérrez-Escudero, R., Pérez-Jiménez M. J., & Rius-Font M.  (2009).??Characterizing tractability by tissue-like P systems.  (Gheorghe Paun, Mario J. Pérez-Jiménez, Agustín Riscos-Núñez, Ed.).10th Workshop on Membrane Computing.  269-281.