prepend(section) {
    // var index = this.spineItems.unshift(section);
    this.spineByHref[section.href] = 0;
    this.spineById[section.idref] = 0; // Re-index

    this.spineItems.forEach(function (item, index) {
      item.index = index;
    });
    return 0;
  }
Prepend a Section to the Spine
@private
@param  {Section} section