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