In a preferred embodiment, {HEAD ~ the bottom} of {TAIL ~ the trough} is of a convex arcuate shape over its full width 
of
In a preferred embodiment , { HEAD ~ the bottom } of { TAIL ~ the trough } is of a convex arcuate shape over its full width
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH