@inproceedings{Green03:tractability,
  title = "Tractability by Approximating Constraint Languages",
  author = "Martin J. Green and David A. Cohen",
  year = "2003",
  booktitle = "Proceedings of the 9th International Conference on Principles and Practice of Constraint Programming (CP'03)",
  series = "Lecture Notes in Computer Science",
  url = "http://www.springerlink.com/content/ewc4fxag1bupg9wh/",
  volume = "2833",
}