Roberto Barbuti Andrea Maggiolo-Schettini Paolo Milazzo Simone Tini A P Systems Flat Form Preserving Step-by-step Behaviour. 1-34 2008 87 Fundam. Inform. 1 http://iospress.metapress.com/content/1866205tn72hu045/ db/journals/fuin/fuin87.html#BarbutiMMT08