_getDiggingDirection(cx, cy) {
        if (cx <= 0 || cy <= 0 || cx >= this._width - 1 || cy >= this._height - 1) {
            return null;
        }
        let result = null;
        let deltas = DIRS[4];
        for (let i = 0; i < deltas.length; i++) {
            let delta = deltas[i];
            let x = cx + delta[0];
            let y = cy + delta[1];
            if (!this._map[x][y]) { /* there already is another empty neighbor! */
                if (result) {
                    return null;
                }
                result = delta;
            }
        }
        /* no empty neighbor */
        if (!result) {
            return null;
        }
        return [-result[0], -result[1]];
    }
Returns vector in "digging" direction, or false, if this does not exist (or is not unique)