Macías-Ramos, L. F., Martínez-del-Amor M. A., Pérez-Jiménez M. J., Riscos-Núñez A., & Valencia-Cabrera L.  (2014).??The Role of the Direction in Tissue P Systems with Cell Separation.  Journal of Automata, Languages and Combinatorics.  19 (2014) 1?4, 185?199.