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