Denis Thérien Regular Languages, Unambiguous Concatenation and Computational Complexity. 49-57 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3328&spage=49 2004 conf/fsttcs/2004 FSTTCS db/conf/fsttcs/fsttcs2004.html#Therien04