@inproceedings{STOC-1974-Robertson,
	author        = "Edward L. Robertson",
	booktitle     = "{Proceedings of the Sixth Annual ACM Symposium on Theory of Computing}",
	doi           = "10.1145/800119.803895",
	pages         = "161--171",
	publisher     = "{ACM}",
	title         = "{Structure of Complexity in the Weak Monadic Second-Order Theories of the Natural Numbers}",
	year          = 1974,
}