@inproceedings{STOC-2005-FortnowST,
	author        = "Lance Fortnow and Rahul Santhanam and Luca Trevisan",
	booktitle     = "{Proceedings of the 37th Annual ACM Symposium on Theory of Computing}",
	doi           = "10.1145/1060590.1060642",
	isbn          = "1-58113-960-8",
	pages         = "348--355",
	publisher     = "{ACM}",
	title         = "{Hierarchies for semantic classes}",
	year          = 2005,
}