subSidebar(level) {
    const currentPath = this.router.getCurrentPath();
    const { cacheTree, toc } = this;

    toc[0] && toc[0].ignoreAllSubs && toc.splice(0);
    // remove the first heading from the toc if it is a top-level heading
    toc[0] && toc[0].depth === 1 && toc.shift();

    for (let i = 0; i < toc.length; i++) {
      toc[i].ignoreSubHeading && toc.splice(i, 1) && i--;
    }

    const tree = cacheTree[currentPath] || genTree(toc, level);

    cacheTree[currentPath] = tree;
    this.toc = [];
    return treeTpl(tree);
  }
Compile sub sidebar
@param {Number} level Type of heading (h<level> tag)
@returns {String} Sub-sidebar element