@inproceedings{STOC-1977-FischerL,
	author        = "Michael J. Fischer and Richard E. Ladner",
	booktitle     = "{Proceedings of the Ninth Annual ACM Symposium on Theory of Computing}",
	doi           = "10.1145/800105.803418",
	pages         = "286--294",
	publisher     = "{ACM}",
	title         = "{Propositional Modal Logic of Programs}",
	year          = 1977,
}