a pair of {HEAD ~ liquid containers} at {TAIL ~ substantially the same level} including a water container and a detergent container 
at
a pair of { HEAD ~ liquid containers } at { TAIL ~ substantially the same level } including a water container and a detergent container
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH