a pair of means each operatively associated with said {HEAD ~ shutter} means for causing said {TAIL ~ shutter} means to selectively close said first and second pipes; and 
for causing
a pair of means each operatively associated with said { HEAD ~ shutter } means for causing said { TAIL ~ shutter } means to selectively close said first and second pipes ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH REL REL OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH