theorem Th20:
  for z being Complex holds z+z*' = 2*(Re z)
proof
  let z be Complex;
  z+z*'=Re z + Re (z*')+(Im z + Im (z*'))*<i> by COMPLEX1:81
    .=Re z + Re (z*')+(Im z + -Im z)*<i> by COMPLEX1:27
    .=Re z + Re z by COMPLEX1:27
    .=2*(Re z);
  hence thesis;
end;
