@techreport{1977-Bergstra-MultiLevel,
  author		= "Bergstra, J.A. and Ollongren, A. and Weide, Th.P. van der",
  title			= "{Multilevel Set Objects}",
  institution		= "Institute of Applied Mathematics and Computer Science, Leiden, The Netherlands",
  year			= "1977",
  class			= "Report"
}