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