Vesa Halava Tero Harju Hendrik Jan Hoogeboom Michel Latteux Languages Defined by Generalized Equality Sets. 355-363 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2751&spage=355 2003 conf/fct/2003 FCT db/conf/fct/fct2003.html#HalavaHHL03