https://9dok.org/document/y4wj52ek-gy-orgygyuriczasupervisor-ferencg-ecsegszeged-abstractoftheph-thesis-characterizationofspecialclassesoftreeautomata-universityofszegedfacultyofscienceandinformaticsdoctoralschoolinmathematicsandcomputerscience.html