@inproceedings{STOC-1970-Lewis70a,
	author        = "David J. Lewis",
	booktitle     = "{Proceedings of the Second Annual ACM Symposium on Theory of Computing}",
	doi           = "10.1145/800161.805155",
	pages         = "100--108",
	publisher     = "{ACM}",
	title         = "{Closure of Families of Languages under Substitution Operators}",
	year          = 1970,
}