@article{BrueningSchaub1999,
  author    = {Br{\"u}ning, Stefan and Schaub, Torsten H.},
  title     = {A voiding non-ground variables},
  year      = {1999},
  language  = {en}
}
@article{BrueningSchaub1999,
  author    = {Br{\"u}ning, Stefan and Schaub, Torsten H.},
  title     = {Avoiding non-ground variables},
  isbn      = {3-540-66131-x},
  year      = {1999},
  language  = {en}
}