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