Kenny Zhuo Ming Lu Martin Sulzmann An Implementation of Subtyping Among Regular Expression Types. 57-73 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3302&spage=57 2004 conf/aplas/2004 APLAS db/conf/aplas/aplas2004.html#LuS04