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