Trellis is a model involving Petri nets, developed by Richard Furuta and David Stotts. See the article in the ACM Hypertext Compendium.
Professor Stotts has detailed information on the Trellis Project home page.
In particular,
look at the first (GIF) image,
Pnet
for moderated meeting protocol
which shows the use of multiple types of tokens and could be of
use for videoconferencing.
