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