The frame may comprise a base frame and {TAIL ~ a operating leaf frame} overlying {HEAD ~ the base frame} 
The frame may comprise a base frame and { TAIL ~ a operating leaf frame } overlying { HEAD ~ the base frame }
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD