    //! Access to a pixel value, using Neumann boundary conditions \const.
    const T& atXYZC(const int x, const int y, const int z, const int c) const {
      if (is_empty())
        throw CImgInstanceException(_cimg_instance
                                    "atXYZC(): Empty instance.",
                                    cimg_instance);
      return _atXYZC(x,y,z,c);
     
    const T& atXYZC(const int x, const int y, const int z, const int c) const {
      if (is_empty())
        throw CImgInstanceException(_cimg_instance
                                    "atXYZC(): Empty instance.",
                                    cimg_instance);
      return _atXYZC(x,y,z,c);