code: def gen_ncx(self, book_idx):
          
          start, end = self._start_end_of_index(book_idx)
          ncx_base = """<?xml version="1.0"?>
  <!DOCTYPE ncx PUBLIC "-//NISO//DTD ncx 2005-1//EN"
   "http://www.daisy.org/z3986/2005/ncx-2005-1.dtd">
  <ncx xmlns="http://www.daisy.org/z3986/2005/ncx/" version="2005-1">
   <head>
   </head>
          <docTitle>
                 <text>{book_title}</text>
          </docTitle>
      <navMap>
          {menavPoints}
      </navMap>
  </ncx>
          """.format(
              book_title=self._title,
              menavPoints="\n".join([chap.gen_ncx(book_idx) for chap in self._chapters[start:end]])
          )
          return ncx_base
docs: 生成NCX文件内容
          :return:
          :rtype: