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