@inproceedings{NACLP-1990-Liu,
	author        = "Yuan Liu",
	booktitle     = "{Proceedings of the North American Conference on Logic Programming, 2 Volumes}",
	isbn          = "0-262-54058-4",
	pages         = "273--288",
	publisher     = "{MIT Press}",
	title         = "{Null Values in Definite Programs}",
	year          = 1990,
}