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