function treemenu(id) 
{  
   main_id = "main" + id;
   child_id = "child" + id;
   main_gif = main_id + "_img";
   display_value = document.getElementById(child_id).style.display;
   
   switch(display_value)
   { case 'block':
     document.getElementById(main_gif).src = "../image/plus.gif";
	 document.getElementById(child_id).style.display = 'none';
	 break;
	 
	 case 'none':
	 document.getElementById(main_gif).src = "../image/minus.gif";
	 document.getElementById(child_id).style.display = 'block';
   	 break;
	 
	 default: return;
   }
}