{HEAD ~ an endless linear means} for moving said cover which is connected to {TAIL ~ opposed side portions} of the cover, 
{ HEAD ~ an endless linear means } for moving said cover which is connected to { TAIL ~ opposed side portions } of the cover ,
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH