Zoltán Ésik, Werner Kuich. A Semiring-Semimodule Generalization of omega-Context-Free Languages. In Juhani Karhumäki, Hermann A. Maurer, Gheorghe Paun, Grzegorz Rozenberg, editors, Theory Is Forever, Essays Dedicated to Arto Salomaa on the Occasion of His 70th Birthday. Volume 3113 of Lecture Notes in Computer Science, pages 68-80, Springer, 2004. [doi]
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