@inProceedings{StraUnd,
   author =    {Stra{\ss}burger, Lutz},
   title =     {System {NEL} Is Undecidable},
   editor =    {De Queiroz, Ruy and Pimentel, Elaine and Figueiredo, Luc{\'\i}lia},
   booktitle = {10th Workshop on Logic, Language, Information and Computation 
                (WoLLIC)},
   series =    {Electronic Notes in Theoretical Computer Science},
   volume =    84,
   year =      2003
}