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