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