@INPROCEEDINGS{CP07-HHM,
    author = "Lin Xu and Holger H. Hoos and Kevin Leyton-Brown",
    title = "Hierarchical Hardness Models for {SAT}",
    booktitle = {Principles and Practice of Constraint Programming (CP-07)},
    series = {Lecture Notes in Computer Science 4741},
    publisher= {Springer Berlin},
    pages = {696–-711},
    year = "2007"
}