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