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