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 = 5;

imgArray = [
  ["../images/kontakt/leider44.jpg","Autograph"," 1"],
  ["../images/kontakt/leider12.jpg","Rosenkavalier, Chicago"," 2"],
  ["../images/kontakt/leider28.jpg","Autograph"," 3"],
  ["../images/kontakt/leider53.jpg","Donna Anna with Richard Tauber"," 4"],
  ["../images/kontakt/leider55.jpg","Postcard, Hamburg ca 1920"," 5"]
]
var imgNumber=0;

function NextImage() {
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    document.images["startimage"].src = imgArray[imgNumber][0];
    document.images["startimage"].alt = imgArray[imgNumber][1];
    document.getElementById("imgsub").innerHTML=imgArray[imgNumber][1];
    document.getElementById("imgnum").innerHTML=imgArray[imgNumber][2];
    return false;
}

function PreviousImage() {
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    imgNumber = NumberOfImages - 1;
    document.images["startimage"].src = imgArray[imgNumber][0];
    document.images["startimage"].alt = imgArray[imgNumber][1];
    document.getElementById("imgsub").innerHTML=imgArray[imgNumber][1];
    document.getElementById("imgnum").innerHTML=imgArray[imgNumber][2];
    return false;
}