{HEAD ~ a recessed portion} therein communicating with said second opening and adapted to receive said {TAIL ~ keeper element therewithin}, 
adapted to receive
{ HEAD ~ a recessed portion } therein communicating with said second opening and adapted to receive said { TAIL ~ keeper element therewithin } ,
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH REL REL REL OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH