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