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