code: function updateTabIndex() {
    var allLinks = document.querySelectorAll('.slide a');
    var position = currentPosition();
    var currentPageLinks = document.getElementById('slide-' + position).querySelectorAll('a');
    var i;
  
    for (i = 0; i < allLinks.length; i++) {
      allLinks[i].setAttribute('tabindex', -1);
    }
  
    for (i = 0; i < currentPageLinks.length; i++) {
      currentPageLinks[i].removeAttribute('tabindex');
    }
  }
docs: Removes tabindex property from all links on the current slide, sets
  tabindex = -1 for all links on other slides. Prevents slides from appearing
  out of control.