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