N. Streitz, J. Geißler, J. Haake, and J. Hol. Proc. Conf. on Computer-Supported Cooperative Work (CSCW), page 345-358. Chapel Hill, NC, ACM Press, New York, NY, (1994)
G. Mark, J. Haake, and N. Streitz. Proc. Europ. Conf. on Computer-Supported Cooperative Work, page 197-213. Kluwer Academic Publishers, Dordrecht, (1995)