    static CImg<T> row_vector(const T& a0, const T& a1) {
      CImg<T> r(2,1);
      r[0] = a0; r[1] = a1;
      return r;
    }
    static CImg<T> row_vector(const T& a0, const T& a1) {
      CImg<T> r(2,1);
      r[0] = a0; r[1] = a1;
      return r;
    }