I. Niemelä, and P. Simons. chapter Extending the Smodels system with cardinality and weight constraints, Kluwer Academic Publishers, Norwell, MA, USA, (2001)
I. Niemelä, P. Simons, and T. Soininen. Proceedings of the 5th International Conference on Logic Programming and Nonmonotonic Reasoning, page 317--331. London, UK, Springer-Verlag, (1999)
B. O’Callaghan, B. O’Sullivan, and E. Freuder. Principles and Practice of Constraint Programming - CP 2005, volume 3709 of Lecture Notes in Computer Science, Springer, Berlin / Heidelberg, (2005)