function zeigen (id) {
  if (document.getElementById)
    document.getElementById(id).style.visibility = "visible";    
}
function verstecken (id) {
  if (document.getElementById)
    document.getElementById(id).style.visibility = "hidden";    
}

function popup(url, target, w, h) {
	l=(screen.width-w)/2; 
	o=(screen.height-h)/2; 
	window.open(url, target, 'scrollbars=yes,resizable=yes,width='+w+',height='+h+',left='+l+',screenX='+l+',top='+o+',screenY='+o); 
}