function clickHandler() {
   var targetId, srcElement, targetElement;
   srcElement = window.event.srcElement;
   if (srcElement.className == "node") {
      targetId = srcElement.id + "d";
      targetElement = document.all(targetId);
      if (targetElement.style.display == "none") { 
         targetElement.style.display = "";
         srcElement.src = "minus.gif";
      } else {
         targetElement.style.display = "none";
         srcElement.src = "plus.gif";
      }
   }
  
}

document.onclick = clickHandler;