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KUNC, Michal. The simplest language where equivalence of finite substitutions is undecidable. In Fundamentals of Computation Theory: 16th International Symposium, FCT 2007, Budapest, Hungary, August 27-30, 2007. Proceedings. Berlin: Springer, 2007, s. 365-375. ISBN 978-3-540-74239-5.Podrobněji: https://is.muni.cz/publication/723254/cs