@inproceedings{STOC-1969-Cook,
	author        = "Stephen A. Cook",
	booktitle     = "{Proceedings of the First Annual ACM Symposium on Theory of Computing}",
	doi           = "10.1145/800169.805437",
	pages         = "229--231",
	publisher     = "{ACM}",
	title         = "{Variations on Pushdown Machines}",
	year          = 1969,
}