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