function establishTopPosition() {
  var aisle = document.getElementById('aisle');
  var y = -20;
  while (aisle!=null) {
    y += aisle.offsetTop;
    aisle = aisle.offsetParent;
  }
    return y;
}
function follow() {
  var action = $("#aisle").children()[0];
  var topPos = establishTopPosition();
  if( window.XMLHttpRequest ) { // IE 6 hates position fixed
    if (document.documentElement.scrollTop > topPos  || self.pageYOffset > topPos) {
      action.style.position = 'fixed';
      action.style.top = '20px';
    } else {
      action.style.position = 'absolute';
      action.style.top = 'auto';
    }
  }
}