parse(xml) {
    var html = Object(core["qs"])(xml, "html");
    var ncx = Object(core["qs"])(xml, "ncx");

    if (html) {
      return this.parseNav(xml);
    } else if (ncx) {
      return this.parseNcx(xml);
    }
  }
Parse PageList Xml
@param  {document} xml