Martin Kutrib Andreas Malcher Detlef Wotschke The Boolean Closure of Linear Context-Free Languages. 284-295 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3340&spage=284 2004 conf/dlt/2004 Developments in Language Theory db/conf/dlt/dlt2004.html#KutribMW04