@unpublished{BaranyTencateOtto2012a,
  year = {2012},
  author = {Barany, Vince and ten Cate, Balder and Otto, Martin},
  title = {Queries with guarded negation},
  note = {Full version of {VLDB'12} paper, 19 pages, available online
          arXiv:1203.0077},
}