Brygg Ullmer Hiroshi Ishii Robert J. K. Jacob Token+constraint systems for tangible interaction with digital information. 81-118 2005 12 ACM Trans. Comput.-Hum. Interact. 1 http://doi.acm.org/10.1145/1057237.1057242 db/journals/tochi/tochi12.html#UllmerIJ05