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