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