Jaco van de Pol Miguel Valero Espada Modal Abstractions in µCRL. 409-425 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3116&spage=409 2004 conf/amast/2004 AMAST db/conf/amast/amast2004.html#PolE04