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