said first and second openings having cross sectional configurations of {HEAD ~ a type} to allow passage of {TAIL ~ said keeper element therethrough}, 
said first and second openings having cross sectional configurations of { HEAD ~ a type } to allow passage of { TAIL ~ said keeper element therethrough } ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH