@TechReport{MargariaSteffen92,
  author =    "T. Margaria and B. Steffen",
  title =     "Distinguishing {F}ormulas for {F}ree",
  institution = "RWTH Aachen",
  year =      "1992",
  number =    "AIB-23-1992",
  url =       "http://aib.informatik.rwth-aachen.de/1992/1992-23.ps.gz",
}