window.onload = function() {
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  if (!document.getElementById("next")) return false;
  if (!document.getElementById("previous")) return false;
  var next = document.getElementById("next");
	var prev = document.getElementById("previous");
  next.onclick = NextImage; 
  prev.onclick = PreviousImage; 
}
var NumberOfImages = 4;

imgArray = [
  ["images/news/leider15.jpg","Bayreuth 1938"," 1"],
  ["images/news/leider49.jpg","Mit Gottfried von Einem"," 2"],
  ["images/news/leider51.jpg","Probenfoto \"Wildschütz\" in Berlin"," 3"],
  ["images/news/leider52.jpg","Opernstudio Berlin 1946"," 4"]
]
var imgNumber=0;

function NextImage() {
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    document.getElementById("imgnum").innerHTML=imgArray[imgNumber][2];
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}

function PreviousImage() {
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    document.images["startimage"].alt = imgArray[imgNumber][1];
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    document.getElementById("imgnum").innerHTML=imgArray[imgNumber][2];
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}