The response level may represent the relationship of the messages within {HEAD ~ the flow} of {TAIL ~ a conversation} in a mail thread 
of
The response level may represent the relationship of the messages within { HEAD ~ the flow } of { TAIL ~ a conversation } in a mail thread
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH