Jerzy W. Grzymala-Busse Xihong Zou Classification Strategies Using Certain and Possible Rules. 37-44 1998 conf/rsctc/1998 Rough Sets and Current Trends in Computing http://link.springer.de/link/service/series/0558/bibs/1424/14240037.htm db/conf/rsctc/rsctc1998.html#Grzymala-BusseZ98