means for supplying two kinds of {HEAD ~ clock signals} having {TAIL ~ the same frequency} and a phase difference of 180 degrees; and 
having
means for supplying two kinds of { HEAD ~ clock signals } having { TAIL ~ the same frequency } and a phase difference of 180 degrees ; and
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH