下面这种a和ref的值一模一样的，要输入两遍感觉很麻烦
能否改为，ref可以为空：ref的值为空的时候，ref的值直接从a那边去取；如果a的值与toc里的索引不一样的时候，再设置ref的值为toc里的索引
这样就可以不重复设置两遍了
navi_items = [
{a = "封面",ref = "封面"},
{a = "扉页",ref = "扉页"},
{a = "版权",ref = "版权"},
{a = "重印前言",ref = "重印前言"},
{a = "序",ref = "序"},
{a = "前言",ref = "前言"},
{a = "目录",ref = "目录"},
{a = "正文",ref = "正文"},
{a = "索引",ref = "索引"},
{a = "参考书目",ref = "参考书目"},
{a = "封底",ref = "封底"},
]