@article{DLT-J-2008-KlimaP10,
	author        = "Ondrej Klíma and Libor Polák",
	doi           = "10.1142/S0129054110007404",
	journal       = "{International Journal of Foundations of Computer Science}",
	number        = 4,
	pages         = "517--533",
	publisher     = "{World Scientific}",
	title         = "{Hierarchies of Piecewise Testable Languages}",
	volume        = 21,
	year          = 2010,
}