Nagl, M., Engels, G., Gall, R., &; Schäfer, W. (1983). 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, pp. 267–287). Berlin/Heidelberg: Springer.https://doi.org/dx.doi.org/10.1007/BFb0000113 
LibreCat: /record/8255
DOI: dx.doi.org/10.1007/BFb0000113

Engels, G., Gall, R., Nagl, M., &; Schäfer, W. (1983). Software Specification Using Graph Grammars. Computing, 31(4), 317–346.http://dx.doi.org/10.1007/BF02251236 
LibreCat: /record/7411
DOI: http://dx.doi.org/10.1007/BF02251236

Engels, G., Pletat, U., &; Ehrich, H.-D. (1983). An Operational Semantics for Specifications of Abstract Data Types with Error Handling. Acta Informatica, 9(3), 235–254.http://dx.doi.org/10.1007/BF00265557 
LibreCat: /record/7412
DOI: http://dx.doi.org/10.1007/BF00265557

Engels, G., &; Schäfer, W. (1982). Specification of a Programming Support Environment by Graph Grammars. In Proceedings of the conference on Graphtheoretic Concepts in Computer Science (WG), München (Germany) (pp. 47–62). München/Leipzig: Hanser.
LibreCat: /record/7811

Pletat, U., Engels, G., &; Ehrich, H.-D. (1982). 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, pp. 254–270).
LibreCat: /record/7812

Engels, G., Pletat, U., &; Ehrich, H.-D. (1981). Handling Errors and Exceptions in the Algebraic Specifications of Data Types. University of Osnabrück.
LibreCat: /record/8184

Pletat, U., Engels, G., &; Ehrich, H.-D. (1981). Operational Semantics of Algebraic Specifications with Conditional Equations. University of Dortmund, Department of Computer Science.
LibreCat: /record/8185