function ch_visible(opt,id_1,id_2) {
	
		if(id_1 != 0)
			document.getElementById('s_'+id_1).style.visibility = opt == 'on' ? 'hidden' : 'visible';
		if(id_2 != 0) {
			if(document.getElementById('s_'+id_2))
				document.getElementById('s_'+id_2).style.visibility = opt == 'on' ? 'hidden' : 'visible';	
		}

}