@Article{kn:EngVer-ContextFreeGraphGrammarsAndConcatenationOfGraphs,
  author = 	 "J. Engelfriet and J. J. Vereijken",
  title = 	 "Context-Free Graph Grammars and Concatenation of
                  Graphs",
  journal = 	 "Acta Informatica",
  year = 	 "1997",
  volume = 	 "34",
  number = 	 "10",
  pages = 	 "773--803"
}