{HEAD ~ The apparatus} further comprises {TAIL ~ an electrical circuit} that automatically shuts off power to the liquid reservoir pump whenever the drain becomes obstructed, while energizing the pressure pump and opening the valve to permit the generation of positive pressure into the drain to dislodge the obstruction 
comprises
{ HEAD ~ The apparatus } further comprises { TAIL ~ an electrical circuit } that automatically shuts off power to the liquid reservoir pump whenever the drain becomes obstructed , while energizing the pressure pump and opening the valve to permit the generation of positive pressure into the drain to dislodge the obstruction
HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH