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