      static double mp_pow0_25(_cimg_math_parser& mp) {
        const double val = _mp_arg(2);
        return std::sqrt(std::sqrt(val));
      }
      static double mp_pow0_25(_cimg_math_parser& mp) {
        const double val = _mp_arg(2);
        return std::sqrt(std::sqrt(val));
      }