the second valve is operable only between {HEAD ~ a valve open position} and {TAIL ~ a valve closed position} 
the second valve is operable only between { HEAD ~ a valve open position } and { TAIL ~ a valve closed position }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL