@TechReport{kn:EngVer-ContextFreeGraphGrammarsAndConcatenationOfGraphs,
  author =       "J. Engelfriet and J. J. Vereijken",
  title =        "Context-Free Graph Grammars and Concatenation of Graphs",
  institution =  "Eindhoven University of Technology",
  year =         "1995",
  number =       "CSR 95/33",
  address =      "Computing Science Department"
}