function popup(alto,ancho,url){
   newwindow=window.open(url,'vent','width='+ancho+',height='+alto+',scrollbars=yes,resizable=no,location=0,chrome=yes,centerscreen,minimizable=no');
   if (!newwindow.opener) newwindow.opener = self;
   if (window.focus) {newwindow.focus();}
}