{TAIL ~ The blades} have {HEAD ~ a retracted position} and an extended position moving across the tunnel 
{ TAIL ~ The blades } have { HEAD ~ a retracted position } and an extended position moving across the tunnel
TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH