Jaroslaw Stepaniuk Relational Data and Rough Sets. 525-539 2007 79 Fundam. Inform. 3-4 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=79&issue=3&spage=525 db/journals/fuin/fuin79.html#Stepaniuk07