@article{1979-Bergstra-Axioms,
  author		= "Goeman, H.J.M. and Ollongren, A. and Weide, Th.P. van der",
  title			= "{Axioms for multilevel objects}",
  journal		= "Fundamenta Informaticae",
  number		= "2",
  volume		= "III",
  year			= "1980",
  pages			= "171--180",
  class			= "Journal"
}