{HEAD ~ an open top trough structure} providing a pooling space in which {TAIL ~ feed material} can pool 
{ HEAD ~ an open top trough structure } providing a pooling space in which { TAIL ~ feed material } can pool
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH