Ian Stark Free-Algebra Models for the <i>pi</i>-Calculus. 155-169 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3441&spage=155 2005 conf/fossacs/2005 FoSSaCS db/conf/fossacs/fossacs2005.html#Stark05