      static double mp_complex_conj(_cimg_math_parser& mp) {
        const double real = _mp_arg(2), imag = _mp_arg(3);
        double *ptrd = &_mp_arg(1) + 1;
        ptrd[0] = real;
        ptrd[1] = -imag;
        return cimg::type<double>::nan();
      }
      static double mp_complex_conj(_cimg_math_parser& mp) {
        const double real = _mp_arg(2), imag = _mp_arg(3);
        double *ptrd = &_mp_arg(1) + 1;
        ptrd[0] = real;
        ptrd[1] = -imag;
        return cimg::type<double>::nan();
      }