Marcelo P. Fiore Mathematical Models of Computational and Combinatorial Structures. 25-46 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3441&spage=25 2005 conf/fossacs/2005 FoSSaCS db/conf/fossacs/fossacs2005.html#Fiore05