In another configuration, the top ply can include a double wall, and {TAIL ~ the bottom ply} can include a single wall which is received within the channel formed by {HEAD ~ the top ply double wall} 
In another configuration , the top ply can include a double wall , and { TAIL ~ the bottom ply } can include a single wall which is received within the channel formed by { HEAD ~ the top ply double wall }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD