function goBack() {
        if (currentIndex > 0) {
          currentIndex--;
          navigateToCurrentURL();
        }
      }
    

      function goForward() {
        if (currentIndex < urls.length - 1) {
          currentIndex++;
          navigateToCurrentURL();
        }
      }
    

      function navigateToCurrentURL() {
        const iframe = document.querySelector('iframe[name="iframe_a"]');
        iframe.src = urls[currentIndex];
      }
    

      function changeIframeSrc(url) {
        var iframe = document.getElementsByName('iframe_a')[0];
        iframe.src = url;
      }