Alfonso Ortega Emilio del Rosal García Diana Pérez Robert Mercas Alexander Perekrestenko Manuel Alfonseca PNEPs, NEPs for Context Free Parsing: Application to Natural Language Processing. 472-479 2009 IWANN (1) http://dx.doi.org/10.1007/978-3-642-02478-8_59 conf/iwann/2009-1 db/conf/iwann/iwann2009-1.html#OrtegaGPMPA09