toc() {
              var b = document.querySelector(".book-toc-expand");
              tocbot.init({
                tocSelector: '.book-tocbot',
                contentSelector: '.book-post',
                headingSelector: 'h1, h2, h3, h4, h5',
                collapseDepth: 2,
                orderedList: false,
                scrollSmooth: false,
              });
              b.setAttribute("onclick", "expand_toc()");
              b.innerHTML = "Expand all"
            }

            function go_t