Steffen Lange, Gunter Grieser, Klaus P. Jantke. Extending Elementary Formal Systems. In Naoki Abe, Roni Khardon, Thomas Zeugmann, editors, Algorithmic Learning Theory, 12th International Conference, ALT 2001, Washington, DC, USA, November 25-28, 2001, Proceedings. Volume 2225 of Lecture Notes in Computer Science, pages 332-347, Springer, 2001. [doi]