François Bry Wlodzimierz Drabent Jan Maluszynski On Subtyping of Tree-Structured Data: A Polynomial Approach. 1-18 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3208&spage=1 2004 conf/ppswr/2004 PPSWR db/conf/ppswr/ppswr2004.html#BryDM04