In effect, the flap and {HEAD ~ base member} define a receptacle within which {TAIL ~ the instrument} can be retained 
In effect , the flap and { HEAD ~ base member } define a receptacle within which { TAIL ~ the instrument } can be retained
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH