public SubGrid (Grid<E> _backing, int _x0, int _y0, int _w, int _h) {
        x0 = _x0; y0 = _y0; w = _w; h = _h;
        backing = _backing;
        if (x0 < 0
         || y0 < 0
         || x0 + w >= backing.width()
         || y0 + h >= backing.height())
            throw new ArrayIndexOutOfBoundsException();
    }