introducing the second member of the first pair and {TAIL ~ the first member} of {HEAD ~ the second pair} into a second vessel from the first vessel 
introducing the second member of the first pair and { TAIL ~ the first member } of { HEAD ~ the second pair } into a second vessel from the first vessel
OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH