Jilles van Gurp Jan Bosch Mikael Svahnberg On the Notion of Variability in Software Product Lines. 45-54 2001 conf/wicsa/2001 WICSA http://doi.ieeecomputersociety.org/10.1109/WICSA.2001.948406 db/conf/wicsa/wicsa2001.html#GurpBS01