Nagl M, Engels G, Gall R, Schäfer W. Software Specification by Graph Grammars. In: Proceedings of the 2nd International Workshop on Graph-Grammars and Their Application to Computer Science, Osnabrück (Germany). Vol 153. LNCS. Berlin/Heidelberg: Springer; 1983:267-287. doi:dx.doi.org/10.1007/BFb0000113

Engels G, Gall R, Nagl M, Schäfer W. Software Specification Using Graph Grammars. Computing. 1983;31(4):317-346. doi:http://dx.doi.org/10.1007/BF02251236

Engels G, Pletat U, Ehrich H-D. An Operational Semantics for Specifications of Abstract Data Types with Error Handling. Acta Informatica. 1983;9(3):235-254. doi:http://dx.doi.org/10.1007/BF00265557

Engels G, Schäfer W. Specification of a Programming Support Environment by Graph Grammars. In: Proceedings of the Conference on Graphtheoretic Concepts in Computer Science (WG), München (Germany). München/Leipzig: Hanser; 1982:47-62.

Pletat U, Engels G, Ehrich H-D. An Operational Approach to Conditional Algebraic Specifications. In: Proceedings of the 7eme Colloque Sur Les Arbres En Algebre et En Programmation, Lille (France). Vol 82. ; 1982:254-270.

Engels G, Pletat U, Ehrich H-D. Handling Errors and Exceptions in the Algebraic Specifications of Data Types. University of Osnabrück; 1981.

Pletat U, Engels G, Ehrich H-D. Operational Semantics of Algebraic Specifications with Conditional Equations. University of Dortmund, Department of Computer Science; 1981.