    double eval(const char *const expression,
                const double x=0, const double y=0, const double z=0, const double c=0,
                CImgList<T> *const list_images=0) {
      return _eval(this,expression,x,y,z,c,list_images);
    }
    double eval(const char *const expression,
                const double x=0, const double y=0, const double z=0, const double c=0,
                CImgList<T> *const list_images=0) {
      return _eval(this,expression,x,y,z,c,list_images);
    }