{TAIL ~ The top ply wall} is {HEAD ~ a single wall} which is received within the bottom ply wall 
{ TAIL ~ The top ply wall } is { HEAD ~ a single wall } which is received within the bottom ply wall
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH