      static double mp_list_spectrum(_cimg_math_parser& mp) {
        const unsigned int ind = (unsigned int)cimg::mod((int)_mp_arg(2),mp.imglist.width());
        return (double)mp.imglist[ind]._spectrum;
      }
      static double mp_list_spectrum(_cimg_math_parser& mp) {
        const unsigned int ind = (unsigned int)cimg::mod((int)_mp_arg(2),mp.imglist.width());
        return (double)mp.imglist[ind]._spectrum;
      }