the leading edge and {HEAD ~ the trailing edge} define {TAIL ~ a first gap} having a first gap width; and 
define
the leading edge and { HEAD ~ the trailing edge } define { TAIL ~ a first gap } having a first gap width ; and
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH