append(section) {
    var index = this.spineItems.length;
    section.index = index;
    this.spineItems.push(section); // Encode and Decode href lookups
    // see pr for details: https://github.com/futurepress/epub.js/pull/358

    this.spineByHref[decodeURI(section.href)] = index;
    this.spineByHref[encodeURI(section.href)] = index;
    this.spineByHref[section.href] = index;
    this.spineById[section.idref] = index;
    return index;
  }
Append a Section to the Spine
@private
@param  {Section} section