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