
theorem
  for x,y being Element of BOOLEAN holds xor2b.<*x,y*> = xor2.<*x,y*>
proof
  let x,y be Element of BOOLEAN;
  thus xor2b.<*x,y*> = 'not' x 'xor' 'not' y by Def8
    .= x 'xor' y
    .= xor2.<*x,y*> by FACIRC_1:def 4;
end;
