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