{HEAD ~ The key lock} can be unlocked by {TAIL ~ a key} only when the combination lock is unlocked 
unlocked by
{ HEAD ~ The key lock } can be unlocked by { TAIL ~ a key } only when the combination lock is unlocked
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH