A distributed index may be used to implement {TAIL ~ an instant messaging system} in a group of {HEAD ~ participating peers} 
A distributed index may be used to implement { TAIL ~ an instant messaging system } in a group of { HEAD ~ participating peers }
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD