parseNav(navHtml) {
    var navElement = Object(core["querySelectorByType"])(navHtml, "nav", "toc");
    var navItems = navElement ? Object(core["qsa"])(navElement, "li") : [];
    var length = navItems.length;
    var i;
    var toc = {};
    var list = [];
    var item, parent;
    if (!navItems || length === 0) return list;

    for (i = 0; i < length; ++i) {
      item = this.navItem(navItems[i]);

      if (item) {
        toc[item.id] = item;

        if (!item.parent) {
          list.push(item);
        } else {
          parent = toc[item.parent];
          parent.subitems.push(item);
        }
      }
    }

    return list;
  }
Parse toc from a Epub > 3.0 Nav
@private
@param  {document} navHtml
@return {array} navigation list