@techreport{Seidl02Expressiveness,
  author = {Helmut Seidl and Aditya V. Nori},
  title = {On the {E}xpressiveness of {T}ree {S}et {O}perators},
  year = 2002,
  series = {Forschungsbericht},
  volume = {02-17},
  publisher = {Universit\"at Trier, Mathematik/Informatik},
}