def extract_tokens(etree_root):
    """
    输出下标 与 字符 的映射关系
    :param etree_root:
    :return: 字典 token_index → token
    """
    mapped_tokens = {}
    for element in etree_root.findall('token'):
        token_index = element.get('t_id')
        token = element.text
        mapped_tokens[token_index] = token
    return mapped_tokens