theorem Th74:
  r |= 'X'('not' H) iff r |='not' ('X' H)
proof
  r |= 'X'('not' H) iff Shift(r,1)|='not' H by Th67;
  then r |= 'X'('not' H) iff Shift(r,1)|/= H by Th64;
  then r |= 'X'('not' H) iff not r|= 'X' H by Th67;
  hence thesis by Th64;
end;
