code: public void back() throws JSONException {
  		if (this.usePrevious || this.index <= 0) {
  			throw new JSONException("Stepping back two steps is not supported");
  		}
  		this.index -= 1;
  		this.character -= 1;
  		this.usePrevious = true;
  		this.eof = false;
  	}
docs: 将标记回退到第一个字符，重新开始解析新的JSON