Udo Pletat, Gregor Engels, Hans-Dietrich Ehrich: An Operational Approach to Conditional Algebraic Specifications. In H.J. Schneider, H. Göttler (eds.): Proceedings of the 7eme Colloque sur les Arbres en Algebre et en Programmation, Lille (France). , pp. 254-270 (1982) 

Gregor Engels, Wilhelm Schäfer: Specification of a Programming Support Environment by Graph Grammars. In H.J. Schneider and H. Göttler (eds.): Proceedings of the conference on Graphtheoretic Concepts in Computer Science (WG`82), München (Germany). Hanser (München/Leipzig), pp. 47-62 (1982)