// OPTIONS = 'width=450,height=450,scrollbars=1,screenX=0,screenY=0';
das_Fenster = null;
function popup(URL, OPTIONS) {
         if (das_Fenster != null && !das_Fenster.closed) {
            das_Fenster.close();
            das_Fenster = window.open(URL,'neuesFenster',OPTIONS);
            das_Fenster.focus();
         }
         else {
            das_Fenster = window.open(URL,'neuesFenster',OPTIONS);
            das_Fenster.focus();
         }
}