* Grzegorz Rozenberg, Arto Salomaa, "Handbook of Formal Languages: Volume I-III", Springer, 1997, ISBN 3-540-61486-9.
* Grzegorz Rozenberg, Arto Salomaa, "Handbook of Formal Languages: Volume I-III", Springer, 1997, ISBN 3-540-61486-9.