Hailang Zuo Mike Mannion David Sellier Richard Foley An Extension of Problem Frame Notation for Software Product Lines. 499-505 2005 conf/apsec/2005 APSEC http://doi.ieeecomputersociety.org/10.1109/APSEC.2005.35 db/conf/apsec/apsec2005.html#ZuoMSF05