@inproceedings{NACLP-1990-GrecoS,
	author        = "Sergio Greco and Domenico Saccà",
	booktitle     = "{Proceedings of the North American Conference on Logic Programming, 2 Volumes}",
	isbn          = "0-262-54058-4",
	pages         = "480--497",
	publisher     = "{MIT Press}",
	title         = "{Negative Logic Programs}",
	year          = 1990,
}