P. Lengauer, V. Bitto, F. Angerer, P. Grünbacher, and H. Mössenböck. Proceedings of the Eighth International Workshop on Variability Modelling of Software-Intensive Systems, page 13:1--13:8. New York, NY, USA, ACM, (2013)
D. Pavlovic, P. Pepper, and D. Smith. Mathematics of Program Construction, volume 6120 of Lecture Notes in Computer Science, Springer Berlin Heidelberg, (2010)