Jan G. Bazan Sinh Hoa Nguyen Hung Son Nguyen Andrzej Skowron Rough Set Methods in Approximation of Hierarchical Concepts. 346-355 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3066&spage=346 2004 conf/rsctc/2004 Rough Sets and Current Trends in Computing db/conf/rsctc/rsctc2004.html#BazanNNS04