theorem
  for a being quasi-adjective of C holds Non a <> a
proof
  let a be quasi-adjective of C;
  per cases;
  suppose a is positive;
    then reconsider a9 = a as positive quasi-adjective of C;
    Non a9 is negative quasi-adjective of C;
    hence thesis;
  end;
  suppose a is negative;
    then reconsider a9 = a as negative quasi-adjective of C;
    Non a9 is positive quasi-adjective of C;
    hence thesis;
  end;
end;
