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