@TechReport{kn:EngVer-ConcatenationOfGraphs,
  author =       "J. Engelfriet and J. J. Vereijken",
  title =        "Concatenation of Graphs",
  institution =  "Eindhoven University of Technology",
  year =         "1994",
  number =       "CSR 94/42",
  address =      "Computing Science Department"
}