Jo Erskine Hannay Shin-ya Katsumata Donald Sannella Semantic and Syntactic Approaches to Simulation Relations. 68-91 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2747&spage=68 2003 conf/mfcs/2003 MFCS db/conf/mfcs/mfcs2003.html#HannayKS03