@article{Witt2005,
  author    = {Andreas Witt},
  title     = {Multiple hierarchies},
  journal   = {Interdisciplinary studies on information structure : ISIS ; working papers of the SFB 632},
  series    = {new aspects of an old solution},
  number    = {2},
  pages     = {55 -- 85},
  year      = {2005},
}