PRESENCE SYSTEM AND INFORMATION PROCESSING EQUIPMENT, DYNAMIC BUDDY LIST GENERATION METHOD IN PRESENCE SYSTEM, AND PRESENCE NOTIFICATION DESTINATION CONTROLLING METHOD AND ITS PROGRAM FOR USE WITH PRESENCE SYSTEM

A presentity service client has a presentity that issues presence information to a presence service and a watcher which observes presence information for other presentities; a presence change rule holding part holds a presence change rule; and a presence calculating part changes the presence information for the presentity, using as a trigger a change in presence information of other presentities, based on the presence information of other presentity acquired by the watcher and in accordance with the presence change rule held by a presence change rules holding part.