{HEAD ~ the external storage} is polled by {TAIL ~ the second application} for the presence of an encrypted electronic message, 
is polled by
{ HEAD ~ the external storage } is polled by { TAIL ~ the second application } for the presence of an encrypted electronic message ,
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH