D. Salber, A. Dey, and G. Abowd. CHI '99: Proceedings of the SIGCHI conference on Human factors in computing systems, page 434--441. New York, NY, USA, ACM, (1999)
G. Abowd, A. Dey, P. Brown, N. Davies, M. Smith, and P. Steggles. HUC '99: Proceedings of the 1st international symposium on Handheld and Ubiquitous Computing, page 304--307. London, UK, Springer-Verlag, (1999)