Paul Gastin Denis Oddoux LTL with Past and Two-Way Very-Weak Alternating Automata. 439-448 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2747&spage=439 2003 conf/mfcs/2003 MFCS db/conf/mfcs/mfcs2003.html#GastinO03