@inproceedings{OttoPiro2008,
  author = {Otto, Martin and Piro, Robert},
  title = {A Lindstrom characterisation of the guarded fragment and of modal
           logic with a global modality},
  pages = {273-287},
  editor = {Carlos Areces and Robert Goldblatt},
  booktitle = {Proceedings of Advances in Modal Logic {AiML}'2008},
  year = {2008},
}