var ie4 = ( document.all ) ? true : false;
var ns6 = ( !ie4 && document.getElementById ) ? true : false;
var ns4 = ( !ie4 && !ns6 && document.layers ) ? true : false;
var prevLayer = "";
var prevID    = "";

function getStyleOf (id){
   return ie4 ? document.all [ id ].style 
        : ns4 ? document.layers [ id ]
        : ns6 ? document.getElementById ( id ).style
        : null;
}

function toggleMenu(id){
   theStyle = getStyleOf(id);
   /* Voorgaand submenu verbergen */
   if(prevLayer!="" && prevLayer!=id){
      prevStyle = getStyleOf(prevLayer);
	  if(prevStyle!=null){
	     prevStyle.visibility = "hidden";
		 prevStyle.display    = "none";
	  }
   }
   
   if (theStyle!=null ){  
      (theStyle.visibility == "hidden") ? theStyle.visibility = "visible" : theStyle.visibility = "hidden";
	  (theStyle.display == "none")? theStyle.display = "block" : theStyle.display = "none";
   }
   prevLayer = id;
}