According to one preferred embodiment of this invention, centerline 60 comprises {HEAD ~ a weld} that joins {TAIL ~ the two layers} of flexible sheet 20 
joins
According to one preferred embodiment of this invention , centerline 60 comprises { HEAD ~ a weld } that joins { TAIL ~ the two layers } of flexible sheet 20
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH