@TechReport{GrafSteffen91,
  author =    "S. Graf and B. Steffen",
  title =     "Compositional {M}inimization of {F}inite {S}tate {S}ystems",
  institution = "RWTH Aachen",
  year =      "1991",
  number =    "AIB-23-1991",
  url =       "http://aib.informatik.rwth-aachen.de/1991/1991-23.ps.gz",
}