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