Manfred Broy The Role of Requirements and Specification in Product Line Engineering. 3-7 2011 SPLC https://doi.org/10.1109/SPLC.2011.68 https://doi.ieeecomputersociety.org/10.1109/SPLC.2011.68 conf/splc/2011 db/conf/splc/splc2011.html#Broy11a