Roberto Barbuti Stefano Cataudella Luca Tesei 67-79 2004 60 Fundam. Inform. 1-4 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=60&issue=1&spage=67 db/journals/fuin/fuin60.html#BarbutiCT04