@inproceedings{STOC-1981-MullerS,
	author        = "David E. Muller and Paul E. Schupp",
	booktitle     = "{Proceedings of the 13th Annual ACM Symposium on Theory of Computing}",
	doi           = "10.1145/800076.802456",
	pages         = "46--54",
	publisher     = "{ACM}",
	title         = "{Pushdown Automata, Graphs, Ends, Second-Order Logic, and Reachability Problems}",
	year          = 1981,
}