Paolo Bottoni Giancarlo Mauri Piero Mussio Gheorghe Paun Grammars Working on Layered Strings. 339-358 1998 13 Acta Cybern. 4 http://www.inf.u-szeged.hu/kutatas/actacybernetica/vol13n4/cikk01.xml db/journals/actaC/actaC13.html#BottoniMMP98