theorem Th76:
  for t, u st t LD-= u holds 'not' t LD-= 'not' u
proof
  let t, u;
  set v = (t '=' u) '=' (('not' t) '=' ('not' u));
  assume t LD-= u;
  then A1: t '=' u is LD-provable;
  v is LD-provable;
  then ('not' t) '=' ('not' u) is LD-provable by A1, Th61;
  hence thesis;
end;
