Krzysztof R. Apt Roman Barták Eric Monfroy Francesca Rossi Sebastian Brand Proceedings of the 6th Annual Workshop of the ERCIM Working Group on Constraints http://arxiv.org/abs/cs.PL/0110012 2001 CoRR cs.PL/0110012 db/journals/corr/corr0110.html#cs-PL-0110012 informal publication