Wojciech Buszkowski Rough Sets and Learning by Unification. 107-121 2007 75 Fundam. Inform. 1-4 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=75&issue=4&spage=107 db/journals/fuin/fuin75.html#Buszkowski07