       res2 = cimg::eval(0,1,1);                    // will return '1' too
       \endcode
    **/
    inline double eval(const char *const expression, const double x, const double y, const double z, const double c) {
       res2 = cimg::eval(0,1,1);                     
       \endcode
    **/
    inline double eval(const char *const expression, const double x, const double y, const double z, const double c) {