@TechReport{DasSti_j2007_a, author = {J{\"u}rgen~Dassow and Ralf~Stiebe}, title = {{Nonterminal Complexity of Some Operations on Context-Free Languages}}, institution = fin, year = 2007, number = {3/2007}, url = {http://theo.cs.uni-magdeburg.de/pubs/papers/TR07-03.pdf} }