Tatsuya Asai Hiroki Arimura Takeaki Uno Shin-Ichi Nakano Discovering Frequent Substructures in Large Unordered Trees. 47-61 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2843&spage=47 2003 conf/dis/2003 Discovery Science db/conf/dis/dis2003.html#AsaiAUN03