function abre_ventana(local,ancho,alto,ascensor) {
    var ventana = "width="+ ancho + ",height=" + alto  + ",left=50,top=50,menubar=0,toolbar=0,directories=0,location=0,status=0,resizable=0,scrollbars=" + ascensor;
    eval("pop=window.open ('"+local+"','pop','"+ventana+"')"); 
}