The upper inside surface and {HEAD ~ the lower inside surface} of the housing define {TAIL ~ a group} of slots 
define
The upper inside surface and { HEAD ~ the lower inside surface } of the housing define { TAIL ~ a group } of slots
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH