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