providing a cap having {HEAD ~ a third lock element} which mates with {TAIL ~ a fourth lock element} on said cylinder; and 
providing a cap having { HEAD ~ a third lock element } which mates with { TAIL ~ a fourth lock element } on said cylinder ; and
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH