@Article{JGAA-281,
   author = {{Ulrik} {Brandes} and {Barbara} {Pampel}},
   title = {Orthogonal-Ordering Constraints are Tough},
   journal = {Journal of Graph Algorithms and Applications},
   year = {2013},
   volume = {17},
   number = {1},
   pages = {1--10},
   doi = {10.7155/jgaa.00281}
}