Roel Wieringa Wiebren de Jonge Paul Spruit Roles and Dynamic Subclasses: A Modal Logic Approach. 32-59 http://link.springer.de/link/service/series/0558/bibs/0821/08210032.htm 1994 conf/ecoop/1994 ECOOP db/conf/ecoop/ecoop94.html#WieringaJS94