var active_layer="";

function exp_menu(symbol,alayer)
{
    if(alayer == active_layer)
    {
	document.getElementById(active_layer).style.display = 'none';
        active_layer='';
    }
    else
    {
	if(active_layer!="")
	{
	    document.getElementById(active_layer).style.display = 'none';
	}
	
	active_layer=alayer;
	document.getElementById(active_layer).style.display = '';
    }
}