Thierry Boy de la Tour Mnacho Echenim Unification in a Class of Permutative Theories. 105-119 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3467&spage=105 2005 conf/rta/2005 RTA db/conf/rta/rta2005.html#TourE05