A double wall is {TAIL ~ a wall} which defines a channel in {HEAD ~ the ply} in which it is formed 
A double wall is { TAIL ~ a wall } which defines a channel in { HEAD ~ the ply } in which it is formed
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH