said output means is further responsive to said third and fourth signal pairs for additionally providing {HEAD ~ an output signal changer} corresponding to {TAIL ~ selected zero crossings} of said third and fourth signals 
said output means is further responsive to said third and fourth signal pairs for additionally providing { HEAD ~ an output signal changer } corresponding to { TAIL ~ selected zero crossings } of said third and fourth signals
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH