In a further variation, {HEAD ~ the top ply} includes {TAIL ~ inner an outer concentric walls}, and the bottom ply can include one or two walls 
includes
In a further variation , { HEAD ~ the top ply } includes { TAIL ~ inner an outer concentric walls } , and the bottom ply can include one or two walls
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH