    CImg<doubleT> eval(const char *const expression, const CImg<t>& xyzc,
                       CImgList<T> *const list_images=0) {
      return _eval(this,expression,xyzc,list_images);
    }
    CImg<doubleT> eval(const char *const expression, const CImg<t>& xyzc,
                       CImgList<T> *const list_images=0) {
      return _eval(this,expression,xyzc,list_images);
    }