Viktor Schuppan Armin Biere Shortest Counterexamples for Symbolic Model Checking of LTL with Past. 493-509 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3440&spage=493 2005 conf/tacas/2005 TACAS db/conf/tacas/tacas2005.html#SchuppanB05