var visible=new Array();	
function set_visible (id, level) {
	document.getElementById(id).style.visibility='visible';		
	visible[level]=id;
}
function set_invisible (id, level) {
	for (i=level; i<visible.length; i++) if (document.getElementById(visible[i]) && visible[i]!=id)document.getElementById(visible[i]).style.visibility='hidden';
}